CC0 would fit perfectly with prims generated by PrimGen.
For the others maybe this should be left to the author if he wants to release it under CC0 or not but I assume it could become mandatory for new prims if authors accept updated LDraw's policy mentioning that all prims must be submitted on PT with CC0 licence.
For already existing ones it would be similar with the migration from CC2 to CC4 so you would need to collect approval from all past authors... or get those prims rewritten by other authors as it is happening now.
For the others maybe this should be left to the author if he wants to release it under CC0 or not but I assume it could become mandatory for new prims if authors accept updated LDraw's policy mentioning that all prims must be submitted on PT with CC0 licence.
For already existing ones it would be similar with the migration from CC2 to CC4 so you would need to collect approval from all past authors... or get those prims rewritten by other authors as it is happening now.