first, some nice freeware at http://www.bartdart.com/
, nice find!
now to your request - i really like this idea alot.
here's what i'd like to suggest for the program, which might be named something like Screensaver Proxy.
it should be a .scr screensaver which would let you specify the real screensaver to run when it is triggered, and then a list of additional programs to run, after some optional additional delay.
and when interrupted, if the additional programs are still running it should ask you if you want them closed or not.
so an example:
i configure ScreensaverProxy as my screensaver, and from its options i set DimSaver as the real screensaver to trigger.
then i specify that after 60 seconds it should run "backup.exe -myscript".
the idea being that when the screensaver triggers and stays active for 60 seconds, thats the go ahead to run the script.
one nice extra feature would be if you could set some flag that said "don't run this more than once every x hours", so that you could limit the exampl above to running the backup at most once every 24 hours or so.