I looked a bit further into getting the Fallout Demo working on the FO2 engine. The demo critter protos are not the same size as either FO1/FO2, and doing a resize by adding bytes didn't seem to work. Letting it use the FO2 protos got past the proto errors, game started, but there was no map, just black space.
So I tried loading the demo junkdemo.map in the mapper, it won't load (error: incorrect map version). But I remembered seeing this same map in FO1, so I copied that over and used it instead. It works! I am able to walk around in the junktown demo and talk to people.
I had to get rid of all the old interface stuff, and a bunch of other incompatible things, but its possible to make this run on FO2 engine. To do a proper "port" that could work IN the fallout 2 game though, the demo protos would have to be cloned and changed to different PID numbers, and scripts would have to be all decompiled and recompiled with new script IDs. Looks like it actually wouldn't be very difficult.