DO NOT USE!
Since working on this project, I have come to the conclusion that using anything but xscreensaver is a very unwise decision (details). I am no longer updating this project and strongly advise against its use (or the use of the upstream branch). Since this program puts you in peril of disclosing sensitive information, I do think its general concept is a Bad Idea™. Imagine your friend bob writes an OTR-encrypted message to you via Jabber, which causes Pidgin to open a new window display the decrypted message.
If you just want to confuse/annoy people, I would suggest having xscreensaver take a screenshot when the screen is locked, which is then displayed by feh, xv or any other program capable of displaying images on X root windows. Another IMHO very nice (though seriously hacky) method is the one I described in this gist: Have xscreensaver spawn an actual Windows XP VM that curious people can interact with, including automatic reset on screen unlocking.
Thanks for checking out anyway!