Yes, WILL gets it. The only Linux screensavers I have on my system are xscreensaver ones written in C, and using the xscreensaver API. How to get xscreensaver to run my Pascal program? That may be trivial, but if it was trivial, someone would tell us how with a very short post.

As I said before, I have never seen a Linux screensaver written in Free Pascal (I've seen Windows screensavers written in Free Pascal, because there we have Pascal translations of the win32 screensaver API). I suspect if it was trivial, it would have been done by now, even taking into account the relatively small number of Linux Free Pascal developers.