I don't see that changes the answer any.
The reality is that if the piece of utilization equipment that has the xfmr in it is listed to UL standards, the same rules would likely apply as if it were a NEC piece of equipment being installed. UL508a would allow for a 15A CB just as the NEC does.
Personally I would not get real excited about the rating of the CB. I am a fan of using the highest rated CB I can most times to reduce the incidence of nuisance trips. I also size fuses that way. Small transformers are infamous for tripping OCPD on the primary side.