Exactly what I was thinking, either for slide shows, or simply serving as a (more interesting) techno-nightlight so I can find it at night without turning the light on (xmatrix, phosphor & (fortune|top), and substrate should serve nicely, at least).

But xscreensaver can actually run any program (no concept-modification involved!), so something like mclock, ciroip's flip clock, or xdaliclock could all work nicely. It should be easy enough even to rig a script that runs, e.g., xv and xdaliclock -transparent for clock+slideshow, but matchbox has the potential to be very much a pain here...