Custom commands aren't too hard to implant, but it requires assembly. I did it in my hack - I have a command that allows me to flash the screen various colors (rather than just white.)
00422510 is where the script parser's main routine goes.
At 00422666, it tests for the "LI+" command and handles it immediately afterwards (by reading an integer parameter from the script and increasing the current health by it) for instance.
Edit - How about CMY, to change Quote into something else, sort of like CNP. Because sometimes, you can't predict where Quote will land and you need to do a cutscene, would be useful, right?