We used make sure that DD sat on the toilet before naps to try and minimise the chance of having an accident. If he is doing it fairly frequently, it's probably worth still putting a nappy on him for naps. Shouldn't impact on the toilet training, just explain that it's just for nap time.

Just re-read and you said that most naps are unplanned - does he just crash somewhere? That makes it a lot trickier if you can't really plan for it, not sure I've got any ideas sorry.