Also, have you displayed the whole palette on the empeg? You can display more than 4 colors but iirc not many more than 4 were useful.

From what I understand, while the display itself supports more than 4 shades, Hijack's sreenbuf is just 2048 bytes, so > 4 shades can't be represented. I think the stuff Toby does in the visuals involves straight assembly to the display which allows more "shades." And from what I've seen, other shades wouldn't be very useful due to too much flickering.

Is ~4 shades of "grey" going to be useful for displaying an album cover?

I think if it scrolled through the album cover vertically, yes. I did a few with empan and they were recognizable, though obviously not photorealistic. To give you an idea of what to expect, if you have emphatic installed, bring up its menu and hold down the knob while the "On" menu item is selected for a couple seconds, and let go. You'll see the emphatic "easter egg" which will show some credits and then scroll an image from the Simpsons. That's about the level of detail I'd expect for album covers.