There has alway been a lot of discussion of PMK not working well in rotary processing, with oxidization being the culprit. But I have used PMK in a Jobo film tube for processing 120 and 4x5 negatives very successfully as well as a print tube for 5x7 and 8x10 negatives. I think I am successful as I use more developer than the minimum.

I can see that a quick shot of nitrogen or other inert gas would help as it would expel oxygen, but don't see how adding more metaborate would help as that is there already in the form of solution B (a heavily saturated solution at that).