As in the hotkeys are limited to what is supplied in the drop down list. For example, one can't define a hotkey to use the windows key and spacebar.
Ok, I just wanted to be sure what you meant. I avoided doing most of the regular letter keys since that would conflict with most editors, but I don't see a reason not to add spacebar.
Edit: I've added spacebar, plus the up, down, left, right arrow keys to the main key list. Hopefully this will provide a few more permutations.
The latest build with the new keys is uploaded to my page.