I've seen several mods wherein the geometry for a bed's frame has been physically separated from its mattress/bedding. I'm not criticizing such mods one way or the other, I just don't understand the utility of it.

I haven't seen it myself, I've heard about it, and it just sounds like LEGO. Put a few pieces together and you have a custom bed, no need for 50 different swatches. If you would have: 5 bed frame parts, 5 mattress parts, and 5 pillow/cover parts, that's 5 * 5 * 5 = 125 different combinations. Sounds pretty sweet.
And if what you're talking about is not what I'm talking about, my idea is doable and probably already has been done.

That pretty much answers it, thanks.

I've seen plenty of separated beds before.

I've heard that there's issues related to MOO (MoveObjects On) with them, though. Is this still the case?

They are done to remove the limitations of only having certain bedding patterns available for specific beds.