I wondered if I had already seen a system like this a long time ago that I subconsciously forgot about because I would have sworn I was just looking at the 2" bottom drain adapters and thought, aha... sanke fermenter. So many pioneers running around in this hobby that it's really hard to be original even when you think you are. Hmm, just to make them really cheap now..
I've been prototyping and experimenting with APRV? Adjustable Pressure Relief Valves built with off the shelf parts. The cheapest junk versions you'll find on Amazon and Mcmaster for around $5 use a circular seat and ball bearing valve. They leak in most of the adjustable range. It's good for not blowing up a compressor tank, but not for something we want to hold pressure indefinitely. The one with the big plastic knob for $13 is much better at holding pressure because the valve has an oring. I've tested using PTFE and MDPE balls for in tapered seats like flare fittings have and that doesn't work at all.
As much as I really do love stainless anything, it's probably not worth the R&D cost when the brass one that works is $13 because it won't touch your beer.