Hrm, this setup could be made to work, I can think of two ways:
1: A freewheeling crank would allow to stop pedaling with the motor spinning, without throwing the chain/ruining the dérailleur.
2: Go SS or hub gears at the back, and have the chain tension dialed in via motor mounts (that bike has vertical dropouts).
I would see this better with a second chainring on the non-drive side, stokemonkey style.
http://clevercycles.com/bike/cp/