Back when I had one of these (it came with the Retina Z2 graphics card in the 3000 I bought used) it did have a video on workbench function. However it was like 5fps tops. Faster than a parallel port frame grabber, but not real motion. The Macrosystem software that came with the card could funnel its output full screen through the Retina. Try watching a movie on it at 4 fps.. It's like a really fast slide show ;-)
I still remember how awesome that computer was when I got it. My PC owning friends thought it was really cool too. It couldn't play Quake like their P200s but it could print at the same time as doing other things ;-) And, wow could it rock Shadow of the Beast!
Good times. I think that's why we're all still here..