Doubling down, not beating a dead horse:

How hard would it be to install a small motor in a rear derailleur upper pulley wheel? Much easier if you've got a battery stuck on there already. The chance of a UCI ban on all bike-mounted electronics is slim, but not zero if they decide mechanical doping is a serious problem.

Edit: immediately realized I had the chain going backwards. That setup wouldn't work. The point that mechanical doping could be more easily detected if all bike-mounted electronics were banned stands.