First, when giving a presentation with an external monitor, I'd like to tag specific windows to be mirrored (and scaled, if appropriate) on the secondary monitor. That way I can still be messing with my instant messages or email in private (which you can't do if you are mirroring your whole screen)

You could start another X server for the secondary display and drive it via vnc or something like that. It's a different approach, but checks most (all?) Of the boxes.