No - and yes. Captivate has no function you can set to do
this. On the other hand, it is possible you can have a third-party
"keyboard activity" program set up to automatically "key" the
PrtScr key every so often.
I haven't tried to do this, but would imagine it could
possibly work. Then again, maybe not, because there might be a
built-in conflict between that software and Captivate itself, over
control of the keyboard ...