A motor on a bike catches the attention of people and starts conversations about different MB designs. An older guy (I'm 45) was telling me that when he was young, they would use vert-shaft lawn motors on bikes with friction drives on the side of the tire. I didn't ask him detailed technical questions. I did a google search and noticed you are researching other MB forums as well. No harm in that.
The rim might seem like the best place to apply the sideways friction, but I doubt it. You could try it and if it doesn't work, then try the tire's sidewall.
Like bandito says, it takes a bit of pressure to keep a friction drive from slipping.
On my bike, I know it is about 20 lbs because I use springs. I used to have one spring on each side, but they were being stretched close to the limit. When one would break, slippage would result, rendering the motor unusable. So I measured the spring tension and modified my setup to have the same tension with 2 springs on each side. I haven't had a problem with it in 2 years.
That 20 lbs of pressure is pressing a roller (my setup is all homemade) made from hockey puck material, downward onto my skinny 10-speed tire - 27 x 1 1/4". A friction drive roller against a steel rim will have to be made of rubber. You would be getting even much less contact area than my setup, and the rubber-to-steel friction would be less than rubber-to-rubber friction. Plus, friction drives probably depend on the "give" of the pneumatic tire.
SO... I don't think there is any way you can get proper sideways friction on the rim. It would be nice - no tire wear - but I'd bet 1000 bucks it ain't practical.
That leaves the friction-on-the-tire-sidewall option. It will work, I'm sure. Fat tires probably have as much rubber on the sidewall as my skinny tires have on the tread. I would definitely only use a rubber friction roller, but I'm sure some will disagree. The wheel ain't gonna like the sideways stress, but you could probably get a few miles out of it.
People here are recommending a horizontal-shaft engine for a reason.