> Back in the PegI prototype days, there were some pics which
> looked to me an awful lot like a ZIF socket CPU (single
> or dual G4) on such an adapter plugged into the Pegasos I.

Yes, you are completely right, these adapters exist(ed?).

Before that there was the ATX pre-PegI prototype board with 2 ZIF sockets onboard:
http://www.bplan-gmbh.de/news/pegasos2.jpg
(one socket occupied by a Sonnet Encore/ZIF G4 card)