From my understanding of RMX-OS, the battle system would probably have to be re-written in order to allow players from different clients to all be part of one party. (It would need to do things like wait for other players to make a command, and so on.) I imagine that this would be possible, but it would probably take a little work.
I think that Blizz-ABS might have at least basic support for this already, but I'm really not sure on that one - I very well could be mistaken. Sorry that I couldn't be of more help.