Unless the rear axle is somehow a freewheel, I would think trying to spin ~3hp worth of motor would have a pretty large braking effect.
It's a cool project, especially for the budget. I have a huge backlog of projects that I've yet to have the time (all work and no play makes Jack a dull boy.) to get started, let alone completed. My damn sig is evidence of this... I planned an e-bike a while back and reading this just bumped it up closer to the top of the to-do-list.
Thanks for the inspiration!