Ok received my bike back for the same reason, three thermostat housings replaced.
Last one was done a week ago everything seemed ok, until tonight went for a ride came home started cleaning the bike for a ride for tomorrow. Just for chits and giggles i checked the coolant tank and surprise the tank is empty.
I did check the tank when i received the bike back from the dealer last week and it was full then.
Now what???
HELP 
Has there been any sign of a leak since you got it back? Any sign of fluid anywhere on the bike itself, like at the heads? If there is no sign of external leakage, and there should be if you really lost a full quart of coolant, I'd start wondering if they purged the air out of the system after the last repair.
Since the current production customers don't seem to be having all these problems that the early production buyers have had, obviously Harley has made several changes in production that have solved the problem, at least for now. So it shouldn't be that difficult for them to just duplicate the same thing by replacing all the parts that have been improved or changed on current production bikes. If they do that, and the dealership doesn't screw anything up when performing the work, it shouldn't be that difficult to get this problem behind you. In the overall scheme of things, this is a very simple cooling system and it shouldn't be that hard to get it right.

Have you contacted H-D directly about these multiple failures, or just the dealership? If you haven't contacted H-D, perhaps you should do so.
Jerry