kj - I've seen pictures of it done two ways. Both were on Stella scooters (which are essentially Vespa PX-150's made in India). One had a modified Yakima or Thule rack mounted on the luggage rack of the scooter and held in place with guy wires. The bike / bike rack stuck way off the back of the scooter, but apparently it worked in the sense it was secure.
The other used some type of aluminum tubing mounted under the floorboard of the scooter - one fore, one aft - and then a Yakimar or Thule (don't remember) bike rack mounted to that. Two issues arose with that for the user - 1, sharp turns to the right could result in the tubing/rack hitting the road, and 2 the bike blocked the right side of the scooter limiting mount/dismount to the left side only.
I used to frequent forum called Stellaspeed but havne't been there in a while. You might want to pop over there and ask -- lots of good mechanics, and they don't just talk/know about Stella's, but all kinds of scooters (mostly vintage or vintage-ish).