I think I know exactly what he was involved in, and have written on the topic before.

I'll repost it here for you:

OK, so focusing on the Far East as that is the core of your question (there is of course also repatriation issues of POWs the Soviets liberated from Germany, but that is a whole 'nother thing, and very different), keep in mind that the USSR was not at war with Japan until August, 1945, and had in fact signed a neutrality pact with Japan in the Spring of 1941 which they honored for most of the war. I've written more about the state of the Far East here which I'd recommend for further background.

In any case, their pact explicitly required neutrality between the two countries when it came to the conflict of the USSR with Germany, and the Western Allies with Japan, as per Article II:

Should one of the Contracting Parties become the object of hostilities on the part of one or several third powers, the other Contracting Party will observe neutrality throughout the duration of the conflict.

This in turn meant that both countries had certain obligations to observe as neutrals. The Hague Convention of 1899, to which both countries were signatories (Japan properly, the USSR as the successor to Russia) would be the most important in this case as it laid out specifically how a neutral power was expected to treat military personnel who came within their borders:

Art. 57. A neutral State which receives in its territory troops belonging to the belligerent armies shall intern them, as far as possible, at a distance from the theatre of war.

It can keep them in camps, and even confine them in fortresses or locations assigned for this purpose.

It shall decide whether officers may be left at liberty on giving their parole that they will not leave the neutral territory without authorization.

So in simplest terms, to have repatriated Allied personnel openly would have been a violation of neutrality. Now, to be sure, as you already note, while they Soviets obeyed the letter of the law here, they absolutely felt pressure from the Western Allies to breach that obligation and return personnel to them. This wasn't something that they could do openly without causing diplomatic issues, but it didn't stop them from, at times allowing an "escape" with a very knowing wink and nod. Edward York and his crew, who had landed their B-25 after the Doolittle Raid, were, as you note, assisted in getting back home via Iran the next year. Held right on the Iranian border, it had been billed as nothing more than an escape, and the Soviet involvement kept secret.

Over the next year, several more bombers ended up in the USSR. After shuffling between several different Siberian locations, Americans started to be permanently interned in at a former school (and before that, apparently, a nobleman's estate) in Tashkent, Uzbekistan. Established as the permanent camp for American internees, which at least was more comfortable than a German POW might expect, it of course was hardly paradise. Although they had space and material for activities such as baseball and basketball, it was nevertheless, in one recollection, "a hell hole", with food often little more than cabbage soup and bread, and their lone blanket little protection against the Soviet winter. Additionally they were treated with suspicion, often interrogated about their missions and other military matters in a way little different than a POW might expect (Especially fascinated by the B-29, the Soviets would reverse engineer it as the Tu-4). At least some crews, apparently unaware of this specific quirk in the laws of war, were quite surprised at the treatment entirely. As one American flier recalled:

The pilot conferred with the engineer and the navigator and made the decision that we would not have enough fuel to get back beyond enemy lines. After reviewing our options, we decided to head north to Vladivostok in the Soviet Union. [...] Most of the crew felt that the Russians would help us repair the Ding Hao and then give us enough fuel to get us back to China.

They were instead quite surprised to be met at gunpoint, and whisked into captivity.

A few attempts at actual escape were made, only to be caught and returned, but plans for further assisted escapes also went into motion, which of course was met with enthusiasm. As one flier recalled getting the news:

The escape was something we all really wanted badly. We had been cold and hungry for months and any kind of a rumor could get us talking about getting out. Unfortunately, the rumors were always just that until late January, when an emissary from the American Embassy in Moscow visited and told our senior officers that arrangements were completed for us to be spirited out of the camp and indeed out of the Soviet Union as well.

With 60 Americans now held by the Soviets in early 1944, the US Army Military Attache, Lt. Col. McCabe arranged for a plan that would involve the internees being taken to Ashkhabad with the stated purpose of flying Lend-Lease Aircraft within the USSR. During the movement of course, an excuse was made to stop the train for the night, allowing the Americans to "escape" into waiting trucks, which drove them to the border and a US Army camp at Amirabad, but not before requiring them to turn over anything on their person which was Soviet in origin.

More of course continued to end up in Soviet custody, and at a much faster rate as B-29s now came in reach of the Japanese Home Islands (previous crews were mostly coming out of Alaska, or the Asian mainland), and following the Second Escape, 101 internees were again the USSR by the end of 1944. McCabe worked to arrange for a similar plan, with a mass transfer allowing to cover for a mass "escape". This time didn't go so smooth though. While the involvement of the Soviets in the Doolittle escape had been kept under wraps for quite some time, and "escapees" had all signed pledges of secrecy, in timing that couldn't be worse the American press was finally breaking the true story, or at least something close enough to it. The first few days of December, 1944, saw newspapers blaring out the AP story with knowing quotation marks that:

The latest of Lieut. Gen. James H Dootlittle's Tokyo raiders to be heard from, the five who landed in Russia, "escaped" across one of the world's best-guarded borders, and have returned safely to the United States.

Afraid that the subsequent escapes would similarly come to light, the Soviets got cold feet literally hours away from the time to make a break for it.

In the train, and sitting on the siding only 30 miles from the border, and waiting for the darkness to cover their break, Ensign William A. King wrote of it:

Everyone started to talk of escape because we are nearer the border than we ever will be again. [...] Little groups started forming [...] it became a matter of mass psychology.

In the end, 34 of the men decided to make the attempt anyways. Most were caught almost immediately, and the most elusive seven made it to the border but in the end were rounded up as well. Returned to Tashkent, they found themselves now treated harsher then before, and hectored that any further escapes would see them placed in the POW camps with the Germans!

That was, however short lived. The lack of a Japanese response to the story, and personal communication from Roosevelt to ensure secrecy would again be maintained, allowed a resumption of the plan in late January, 1945, this time being carried out without a hitch as the group, which now numbered 130, were easily scooted to the border and back into American custody., although their trip home would not be uneventful, then their ship the Sullivan encountered U-Boats on the trip from Oran, Algeria, even if it never came under fire. Two further escapes would be carried out for later batches of fliers as well, of 43 and 51 respectively, the last group, ironically, leaving the border on August 24, when the war was essentially over.

Sources

"5 Doolittle Fliers Flee from Russia". 1944. New York Times, Dec 03, 1944

Hays, Jr., Otis E. “Lost & Found in Siberia.” Russian Life 46, no. 2 (March 2003): 34.

Heberling, Michael & Jack Schaefer. "Superfort Crew's Siberian Odyssey" Aviation History. 21, no. 2 (November 2010) 38-42

Larson, George A. “American Airmen Held as POWs in Far East Russia During World War II.” Air Power History 59, no. 2 (June 22, 2012): 24–31.

Sears, David L. "Pipeline to Freedom". Naval History Magazine 31, no. 1 (February 2017)