A standard wooden hive has conductivity of 6w per degree C and the bees need at least some of them to be at 34C, The bees have no choice but to cluster as it would take 100s of kilogrammes of honey to get through winter if they heated every bee and the volume of the hive to 34C. (yes i have done the maths)
A tree hollow has a conductivity of somewhere between 1 watt per degree C and 0.3w per degree C depending on the thickness of solid and rotten wood. Now in such a tree hollow the possibility exists for the bees not to cluster. If the bees use 16kg of honey in a winter, this could average out at ~20Watts . 20watts in a tree hollow could heat the the entire hollow to 34C even if it was well below zero C outside. Can bees survive in such a warm hollow? I think they have done so for thousands of years Do they cluster in such a hollow? probably not. However, we can conduct an experiment to find out if they do cluster even when there is no themal need.
At this moment I have a colony in a 0.4W per Degree C hive. They were split into that type of hive, and the queen has been in such a hive from egg onwards. The hive is bottom entrance, with a floor that has a plastic coated foam grill with about 15% open area. This grill is heavily shielded from draughts and cooling from below but allows ventilation and trash to fall down and out. This floor is completely unlike any other you may have seen. in addition the entrance is designed to shield the hive floor from drafts.
The difference in the behavior of the bees compared to a conventional hive is remarkable. How they get on this winter we shall see.