Ideas

[SleepKiller]

step 1: find something sticky
step 2: stick it to the center of your screen
Wow! now you have a crosshair for ALL your weapons, including sniper!
What you don't have, though, is the ability to quickly or automatically change the position of anything you have sticking to your screen.

Yes I've heard sevreral people doing stuff like that. (The best one being whiteboard marker.)
But most people would not go to the bother for a crossair that will miss some of the time anyway.