I've heard that "hunting accident" is code for a monarch or noble having an undesirable son quietly disposed of (or more generally assassination). Is this true?

It's not "code", but such claims could be met with suspicion. Hunting accidents were both a natural and common occurrence, but also a good cover.

Let's look at one of the more suspicious hunting accidents - the death of king William II of England in 1100. He was out hunting with two men - Walter Tirel and Henry (William's brother and heir to the throne) - when he was shot with an arrow after Walter mistook him for a deer. Obviously, one has to wonder how the hell someone mistakes their king for a deer. Other versions of the story have Walter know where the king is relative to the deer, and is simply unlucky with his shot. This is all difficult to believe because Walter Tirel was known for his skill with a bow and being a keen hunter - that's why the king had invited him to go hunting in the first place! Walter immediately fled to France and denied that he had ever gone hunting with William II. It was also recorded that William II had received a message that morning from a monk who had dreamt last night that the king would die if he took part in the hunting trip, which sounds a lot like a coded warning, but could also just have been a coincidence or an invention to help explain why God had chosen to kill such a sinful king, as in other versions of his death it was William himself who had the dream. It also just so happens that the only other person on the hunt, Henry, was then crowned as king of England and other members of the wider hunt benefited enormously from Henry's rule. If it was an accident, it was incredibly convenient for Henry.

But the strange thing in this case is that people generally accepted it was an accident despite obvious problems with the story. Here's the most detailed account from William of Malmesbury:

“The sun was now declining, when the king, drawing his bow and letting fly an arrow, slightly wounded a stag which passed before him… The stag was still running… The king, followed it a long time with his eyes, holding up his hand to keep off the power of the sun’s rays. At this instant Walter decided to kill another stag. Alas, [Walter's arrow] pierced the king's breast. …Walter immediately ran up, but as he found him senseless, he leapt upon his horse, and escaped with the utmost speed. Indeed there were none to pursue him: some helped his flight…

Although there were rumours that it was an assassination rather than an accident, such voices were not the majority. The fact is, hunting accidents were normal. For prey like deer, a well placed arrow was usually sufficient for a kill, but sturdier animals like boar (a favourite of the aristocracy) had to be fought up close with spears and swords. Getting up close was the manly way to hunt, and that could be dangerous. Another of William II's brothers, Richard, had died in the same forest after being knocked down and trampled by a stag. The great Byzantine emperor Basil I died after his clothes were caught in the antlers of a deer and he was dragged by his belt for miles as it bolted. King Fulk of Jerusalem had the most brutal of accidents - his horse tripped at speed, leading Fulk to fall off and be followed by the tumbling horse. The saddle crushed his skull, but didn't kill him immediately. His injuries were so bad that William of Tyre records how his brains were partially forced out, and he died comatose three days later. Even accidental shootings could occur, such as with Valdemar of Denmark who, despite being very popular and having no enemies who would want him dead, was killed by a stray shot whilst hunting.

So hunting was dangerous, and hunting accidents were a common cause of death that not even kings and emperors could escape. When we hear of some noble dying in a hunting accident, there's a good chance that it actually happened. This is especially true of accounts where it was an animal, not a person, that killed them. Where someone was killed by an arrow, that's when the suspicions start. Accidents could and did happen, but that's what made it a good cover story for a murder. At the very least, a hunting accident created plausible deniability - despite the clear problems with William II's death being an accident, it remains genuinely possible that it was so and the evidence of an assassination can all be dismissed as circumstantial. Given that William II was deeply unpopular, it may have been more convenient to go along with that story than to raise questions. We have no way of knowing whether William II's death really was an assassination.

But generally speaking, 'hunting accident' was not code for 'quietly murdered', even when we have good reason to believe that someone was indeed assassinated.