How about non-exit nodes. I'm actually working on an app (Bitcoin-related) that receives commands from satellite-connected devices (very low bandwidth) and then relays those over the Tor network to a processing server hosted by the user. So this would require a Tor node on the server to get the satellite request and forward it over Tor.

Would this be an acceptable use? Bandwidth requirements would be extremely low (I would say less than 100MB/month in 99% of the cases).