It wasn't just land troops, so sorry that this doesn't address your question directly, but is related. Quite a number of volunteer American pilots flew with the RAF. This is quite a good article about them. There were less than 10 known American pilots in the Battle of Britain, but since one of those was only discovered posthumously (he had claimed to be Canadian), there could have been more.

By the time that the USA entered the war, there were three squadrons of American pilots flying in the RAF. When America entered the war, these squadrens were handed over from the RAF to the USAAF. They became 4th Fighter Group.

While the Neutrality Acts did prevent American people and businesses from doing certain things (mainly trading arms with countries at war), I can find no description on the government history site that the law prevented Americen citizens from volunteering to fight for belligerent nations. Maybe somebody else with more knowledge can clarify the legal aspect of their doing so?

Thousands of American citizens volunteered to fight fascism during the Spanish Civil War with the Abraham Lincoln Battalion. I believe that some German Americans citizens also volunteered for the Whermacht and that some served in the Waffen SS.

I cannot tell you how this affected their draft status. They obviously were already serving somewhere else. The Americans who fought for the Spanish government or for the British faced no sanctions after the war that I know of. Some of those who served in the Waffen SS were prosecuted after the war. However, according to Frank Joseph

Dr Joseph Goebels was diligent in destroying all records of individuals who volunteered from the United States or England [presumably meaning the British Empire and its colonies]

He does describe the prosecution of Martin Monti, who had joined the SS. But it should be noted that he actively deserted the USAAF, tried to join the Luftwaffe, and then settled in the Waffen SS. So he was technically a traitor after his enlistment in the American forces.

Has this situation come up in any other American wars?

The closest case I can think of is Lewis Millet. He wasn't drafted, he enlisted in the National Guard in 1938 and then went on into the Army Air Corps. He deserted in mid-1941 to join the Canadian Army so he could fight the Nazis when it wasn't clear if the US would ever join the conflict. He went with his Canadian unit to the UK to serve as a anti-aircraft radar operator. When the US joins the war he turns himself in to the Embassy and then goes back to the US Army where he serves in North Africa and Italy. In Italy, he gets court-martialed for desertion, his punishment is a loss of leave, a $52 dollar fine. Within a few weeks he gets a field promotion to second lieutenant. In one of Lewis' obituaries, his brother is quoted as saying “He didn’t give a hoot about the leave privileges because he wasn’t going anywhere anyway, but he was a little annoyed about the 52 bucks. and "They told him they had to do that so they could promote him.’’ Lewis Millet went on to serve in the Korean War, where he receives the Medal of Honor for leading a bayonet charge, and the Vietnam War. His final rank at retirement was colonel.

http://www.boston.com/bostonglobe/obituaries/articles/2009/11/19/lewis_millett_awarded_medal_of_honor_after_bayonet_charge/?page=2 https://en.wikipedia.org/wiki/Lewis_Millett

I can only give examples from my own family for what it may be worth. Most emigrated from Canada to Michigan in 1850s, fought in Union Army then moved to Washington state. Timber workers. Several joined the Canadian Army in Victoria, B.C. assigned to 44th Battalion, 88th Regiment Volunteer Force in early 1915. They had to make an oath to "bear true allegiance to His Majesty King George the Fifth,His Heirs and Successors". One of them I knew quite well, he was eventually in the 15th Canadian Machine Gun Company and wounded in 1917, invalided out, married a lady in London and returned to Washington. Then he got a U.S. draft notice in 1918 but failed the physical because of shot off toes. Anyways, he never got in any trouble for enlisting in Canada's Army, nor did any of my other relatives, I don't think he would have cared much, he gave his occupation on the Canadian attestation paper as "soldier". None had any problems with U.S. citizenship, it was never brought up by any U.S. official. None had any problems with U.S, officialdom over serving in Canadian Army or swearing the oath to King George, nor did they lose their U.S. citizenship.

Why would fighting for an ally be criminal?