"I think most applications only respond to keystrokes when they have the keyboard focus."

i hear you, but the one contrary example is the excellent Syphon Recorder which hears the Alt-R-Cmd calling above the clamour of all the other software. Which is seriously useful...

i'm very happy with the spacebar as a quick scene change keystroke, but I want to use it while controlling a different software.

If you absolutely want to stick to keystrokes, there's probably some software out there that you could setup to run in the background which could convert specific keystrokes to MIDI or OSC and send it to Isadora, but there's no native way to do it purely using Isadora because Keyboard Watcher actors and Isadora's Go Triggers based on key presses can only work so long as Isadora is the frontmost application/the application that has the keyboard focus.

I could be wrong, but I think most applications only respond to keystrokes when they have the keyboard focus. To me, it seems like a security risk to allow every software to monitor your keystrokes outside of the program itself, as that's basically what malicious keylogger programs do.

I'm with Fred on this though. I'd use some sort of controller and MIDI or OSC.