I'm looking at the Rheem RTE3, basically the smallest electric model I could find. 120v, minimum flow of 1/2 gallon per minute and an external temp control. I'm gonna guess that 1/2 gallon per minute is the ballpark minimum of anything out there.

The gas unit we had was useless even for the shower, with the new water-conserving shower heads, have to turn on hot water in the sink in addition to the shower to activate the heater. Ended up with an electric 5 gallon tank in the darkroom, which I was not totally happy with, particularly in cold weather.

This Rheem is designed for a bathroom sink or the like, and seems like it would provide 70-80f without wasting too much water or electricity. I think it has a 30 amp breaker, so you could probably fashion it to be able to plug into the wall instead of hard wiring.