I've found with rabbits that consistent temperature is perhaps more important than sticking to really strict guidelines. They can be fine down a little past freezing, as long as they are not being taken back and forth between hot and cold. More so if the rabbit is generally living in 30-40 degrees, going to 70 degrees for an hour or two then back out is quite a shock. I'm betting a garage in western Washington seldom if ever drops below freezing, especially if it is at all insulated. I keep a very close eye on the thermometer in the space my rabbits are inhabiting right now. If I were you, I'd keep a closer eye in the summer months, it may get hotter in there than you think.
Overall I think it could be a fabulous idea! A ceramic heater is also fantastic if you need to cut the cold, although be very careful of fire hazards!