I've actually seen what could be considered a variant of kotegaishi that doesn't even require you to keep your thumb in between their last two knuckles...its sort of hard to explain, and I've only learned it from Ikkyo...imagine you've got someone's arm in ikkyo, but you can't go any furher. So what you do is put the arm that is supporting their elbow over uke's arm, then under to grasp your own wrist. Keep this tight, and twist towards uke. This should, if done correctly, result in a kotegaeshi that doesn't actually require their hand to be in any special position, since it is entirely based on their wrist and elbow. The turnover is a little tricky on this one, but its a nice technique. I think it could be used against a low punch omote, if I were smart enough to figure it out.
If I don't make sense, maybe one of the other posters here can describe it better.