First, enable "Multibyte Editbox" in Alt+O > IRC > Messages and restart mIRC. This is supposed to make the editboxes "normal" (I don't know in what way, though). If you're still having problems I suggest you to contact a guy called
wakjah on Undernet. Recently he wrote a DLL that can manipulate channel editboxes (hopefully he doesn't mind me mentioning it here

).