It's an interesting topic, wish I had good answers for you.
I've been looking at and thinking about absorption cooling system for a while now, that's the process that propane refrigerators use. Getting your head around the thermodynamics of it is surprisingly tricky.
I'm guessing that a heat pump in the cycle you're talking about sends coolant out at say, 0 degrees F and has it returned at 27 F. If the pressure in the system is right which by design, it would be, at that temp. some condensation would take place, which would (should) give off heat.
I can't quite get my head around it. I can figure it the other direction, but in reverse it clouds up.
Howstuffworks claims that in climates that spend time below freezing, a heat pump is not necessarily more efficient than other types:
http://home.howstuffworks.com/how-to-troubleshoot-a-heat-pump.htm