I'm pretty sure Mr purple kale doesn't care about being left outside. They all seemed content in the cold, though their growth is very slow. Likely faster in the greenhouse!
Had to look up the thermal mass system to fully understand what you mean and wow! Looks fab. But like you say, not too much room left in my greenhouse for a load of water. I could potentially move a few plants to make way for buckets. Let's see what kind of temperatures we are getting in the depths of winter. For now it is very mild and it probably needs more ventilation!
I was thinking to put a thermometer in there which records highs & lows. Ideally I would have two in fact, one inside, one out. This way I will know for sure how effective it is. But perhaps the bucket of water comparison is simpler! If the one outside freezes while the inside one doesn't, job done.