This is a great idea! And it works quite well, congratulations Skrommel!
Only one suggestion: why not have key work as a toggle, instead of forcing it to be pressed.
I mean, the user could activate it with a hotkey, like F8, then, the labels appear, and one could press the number on the label. If the user didn't want to activate anything, he'd press F8 again and the labels would disappear.
This would avoid interfeerence with program's hotkeys, and in my opinion, would be a bit more handy, since holding ctrl and pressing a few buttons is not very ergonomic.