Mux, would you consider the option to allow communication with previous renters, at least to some small extent?
I had stability and performance issues with my machine and ended up wasting a guy's time, but he was really nice.
I got my machine hashing to a point that I can deliver on what cgwatcher is actually saying.
I'd like to throw the hours to that guy which he originally set out to rent

Agreed, I have a renter who should have either a free hour or a partial refund, however, now that the rent is over, it is very difficult to contact him/her :/