Probably pretty well known, but uix86.dll can be replaced in the /base folder w/o conflicting with online play. I placed my updated dll there and was able to play online.
Well, the FFA server I joined, |JW|, said 'pure', but I just noticed they were running a mod (checked directories). I'll have to do more tests.
Well, I've found that pure servers seem to allow some unpure data (like menu files and such) if you join the server right after you boot up the game. But, it only works for a while. :|
I'm not sure, but I will tell you that MAX_PLAYERMODELS is defined in ui_local.h, so that might have a lot to do with it.