I'd love to see your setup - not that I would ever put an electric assist motor on my bike, but, I sure would be curious to see how yours is setup.
As to your question, what would be so terrible about placing a FD on the bike even if you choose not to attach a cable to it? Position it where it can serve as a guide to keep your chain from jumping off. As they say in the infomercial, "set it and forget it."
It would seem to be a quick, simple solution to your problem using materials already in existence designed for the (well almost the) purpose.
Tell us more about your assist motor. How do you power it? How do you replenish that power?
I admit you have raised my curiosity.