This was something that I was looking for a long time and finally found while looking for a completely different thing altogether!
Agreed I always wanted this only for mischief :P Basically what I wanted was to fire up any GUI so that it would show on a remote desktop and not on my local display. clearly I didn't want vnc or ssh -XCYP.

What I needed was this:

ssh -X host
export DISPLAY=:0

and then I can run any gui and it will open on the remote desktop.
would make things look like some hacky mischief ;)

okay, another thing I learnt was how to start and stop gnome screensaver remotely.

so instead of doing a killall -9 screensaver, you can do
gnome-screensaver-command -d

So there are things that are still fun!
Gosh, do I miss college!