It would be perfect if there was an option to rate a thread while replying and only when replying. If the user has already rated a thread then they wouldn't get an option to rate on a subsequent reply.
Hmm, where do you think they would see the option? You mean if they are in the advanced editor and not in the quick reply they'd see the option to select the rating? And when they're viewing the thread they'll only be able to view the rating?
I was thinking right above the quick reply box and advanced editor if applicable. and yes, the voting stars would be removed from the top rating display in this case.