Hey, Jay,
I wanna meet your boss and compare war stories. I also have changed the charging valve on a 600t. "bench grinder" years ago ---------while it was runnng. Worked like a champ. Whooped once and kept going. Had to do it on the fly because, like your boss, the problem was, even though he had a vacuum, refrigerant is HEAVY. His static head was more than the vacuum level. I needed to change the valve to recover the refrigerant. Had a "catch-22" situation. I knew I didn't have enough vacuum to hold back the head of R-11 charge if the machine wasn't running.
I've changed 19D oil filters and even an oil pump by maintaining pressure/head relationships. Absorber Techs. do it all the time.
Back to the origional post. No, you cannot reliably or safely do a braze repair with anything more than a whisper of pressure or vacuum.
"Wheel" mechanics work on "Wheel" chillers