Hmm, yes. I was envisaging that the two new keys would need to be unique against all others, like other key settings are; but actually it probably makes sense that you want to reuse your ok key as a game control key when not in a chat box. If the game considers the ok/cancel mode and the game mode as being modal, keys only need to be unique in their specific mode.
Well I suppose clearing the text box when you open the chat window in game, and making it so that it ignores game controls while in the chat window could work to a certain extent - although it would stop me configuring space as whichever game control it uses as cancel (fire, I think - if not jump). But it would mean I could use the whole keyboard for things other than fire and jump without them cluttering up the chat box (like I currently have power on , and . because my keyboard doesn't have dedicated pgup/down keys).