Shower trim is designed to get wet, so I don't think that there would be a problem. If there is any possibility of freezing, you need to make provisions to easily drain the system, or bad things will happen. Since the line to the shower could be a long ways away, you might suggest a hot water recirculation system. Maybe one switched, rather than on all the time to save some energy. You might even be able to hook it up to a motion sensor so that when someone walked into/towards the shower, it turned on the pump.