IIRC, the electrical power from the internal winding is actually a generator with a permanent magnet and does not have a field winding to modulate. (That's why we have a lot of power to dissipate in the external regulator and the resulting lifespan issues with those regulators.) It may make sense to rely on the internal 912 generator as primary with the external unit kicking in at 0.5 V below your regulated output.
(That's my understanding of the 912ULS. The 912iS has a different system. I belive it has two independently wound generators on the same rotor.)