So, I cleaned up the bench and did a test. Turns out I have a RV622A (100 kohm input), not a 722. Used that against a North Atlantic Industries RB-504 ratio box (ratio transformer). Various "null" detectors were tried: HP3478A DMM, Tektronix 1A7A differential plug-in and a floating GR 1232A AC bridge null meter. Signal was from a Wavetek 185 at 50-400 Hz.
The short answer is this was a dismal failure. The phase shift problem is so severe that one can only null about 3 digits, not exactly metrology lab precision. Just being read with a DMM, either box divides as expected. The easiest thing to do would be to simply calculate the error on on the RV unit due to loading and factor that in. The impedance of the transformer box is low enough that it can be ignored in many/most cases.
I'm going to try the phase compensator shown in the manuals and see how well I can do with that. There's still the matter of capacitive loading of the RV box, so one has to factor that in, depending on frequency. Next test will just use the KVD and see how bad the loading problem really is, and if it's easy to compensate.
There are some GR articles, one in the May 1968 Experimenter, talking about how they could achieve 200 ppb with the ratio transformers. I think I've got a ways to go yet.