The Serwa 4 stroke auxiliary motor used 2 drive rollers at different speeds on a rocking mounting, running on the inside face of the drive rim. Weird, but then it's French, which explains everything.
I took a second look and it seems the engine can pivot to engage the rim which seems to be bolted onto the spokes. Couldn't figure out what that 2nd roller was for but the 2 speed idea is genius.
You could adapt the idea for a rack mount engine if you wanted. Or even 2 flat sections of roller with a ramp face between the different diameters, and push the engine sideways across the tyre.
Yeah I've always thought it'd be an awesome idea to for a race bike to rack mount a powerful lawnmower engine with a vertical shaft that friction drives on either side of the tire. The tire wouldn't last long but that's why it's a race bike! It would also allow you to use huge 22+ horsepower lawn tractor engines. Just thinking I guess.