I doubt that (m)any people have tried such a thing. Given the fact that it is almost always undesired behavior for any unfocused application to respond to input, I would think that you'd have to insert some kind of 'hotkey' hook into the os interrupt handler that would send messages to the background application in question. This is probably infeasible with Construct. Perhaps it's possible with the aid of Python.
I could be way off base, I suppose...