Okay, I know I'm totally obsessed, but it occurs to me that doing this wouldn't be totally unfeasible.
As far as I can see (and I'm no expert), there shouldn't be any reason I couldn't get my hands on a G4 Mac mini quite cheap, and put it along with a 15" LCD screen inside the casing of an iMac G3. As I said, though, I'm no expert, so I could be way off the beam here.
Anyone have any suggestions or observations on the possibility of doing such a thing? As far as I can see, the hardest parts would be powering them both inside the case from one cable (it may not even be possible), connecting up the mini's USB and FireWire ports to those on the outside of the iMac, and aligning the optical drive correctly so it can be accessed through the slot on the front.
Anyone who has any suggestions or observations, I'd love to hear them.