Hm... I don't like this idea. =/
First off, battles are rarely ever replied to that quickly, and I don't think that they should be. Essentially, your system FORCES people to stay in front of the computer and battle, or forfeit the match completely by closing the chat window. This already takes out half of the RPers, the half that posts a move, and then goes away to do other stuff in real life, which is the case with a lot of members when it comes to battling, that's the entire reason why battles span over a long amount of time. The reason being because people have other things to do, and this system makes them stay for the entire duration of the battle, which could possibly be hours. There isn't much of a problem at all with spamming and flaming, as the only battles that end up with spamming and flaming in them are the battles that either don't start at all or battles in which the actual fighters are spamming/flaming, in both cases the fight is already rendered completely pointless. Actual fights in which people are actually posting their moves are very rarely subjected to intense spam (maybe one or two posts max, but nothing that throws off the flow of the battle). Second, I don't exactly understand how you expect this to work. =/ The mods would have to program EVERY new chat window. The reason being is because they can possibly make it preset the entire section so that the maker of the thread is the only one who can post, and they can preset the section so that mods can post anywhere, but how would they preset it for the other fighter(s) who aren't making the thread? =/ I don't know if this is exactly what you meant but if it is then they would have to go to each thread and edit it so that the other fighter(s) can post as well. It might work if no one else could view the chat, but the fact that anyone can view the chat, anyone can make the chat, but only specific people can post in it makes things complicated.
I don't really see anything wrong with this specific area of the RP in the first place (this area as in how the battles are done), so I have to disagree.