Just to follow up, I was able to successfully clone an existing installation of 10.6.8 from a 2006 iMac to a 2011 iMac, retaining the built-in mosaic screensaver. I then updated software on the new 2011 iMac one at a time (10.6.8 -> 10.7.5, iPhoto 7.x to 9.x), all while ensuring the mosaic didn't disappear. At this point all is well. Between Snow Leopard and Lion, I did notice the way the 4.x mosaic animation drifts outward has a different easing pattern than the 3.x version. Not a bad thing, just different than we're used to.
Anyway, I saved a copy of the 10.7.5 installation and then copied it to a new SSD drive, updated that to 10.8.5, replaced the required screensaver files, and the mosaic screen saver continued to work. Success!
Unfortunately after upgrading to 10.8.5 the iMac kept freezing on wake from sleep. I couldn't immediately solve that issue after several attempts, so had to downgrade back to 10.7.5, not the outcome I wanted but at least mosaic works and it's not on 10.6.x anymore.
Just posting here in case anyone wanted to know the step-by-step process, if doing a clean install like I first tried didn't work.
