There is one on ebay- a suntour for the PEDAL side, I had the link a week ago but my computer crashed and I reformatted. It's under BMX parts- search "chain tensioner"-
with one on the pedal side, you can set the motor chain and this will adjust the other- if you hardly ever pedal- this would be better, no?
i think it's about $22- and as listed for BMX it probably accomodates a 1/8" cruiser chain. Not sure about 3/32 derailleur chain. But it looks like a little thin derailleur with a hanger but no body and the double pulle wheel under that spring loaded.
I'm goin looking!