Is a main disconnecting means needed on the primarary side of a 480v transformer with a 120/208v secondary if the transformer is 150' away from the MCC?

I think there are some missing pieces here. Describe what's connected to the secondary and describe in detail the setup.

I'll just mention that there seem to be many rules lately relating to disconnects being lockable in the off position when they are far away from the equipment.

I still may not be able to answer this question, but there is someone here that should be able to.

A fusible disconnect or circuit breaker is needed to protect the primary feeder conductors and the transformer, but does not need to be located at the transformer, but at the beginning of the feeder to the transformer, in your case, at the MCC.