Hey folks,
I'm also fighting with the DeckLink Mini Monitor 4K and 12 bit RGB currently but just fixed the bug while looking at my code when writting this. I can say:
It works.
There are two small tools I play with using the DeckLink SDK to output video via SDI and on both I have different formats implemented, one of them being bmdFormat12BitRGBLE for 12 bit and I get output to the VideoAssist 4K I have currently on my desk. Tested with 1080p24 and 1080p60 and the image looks right.
PS: And normally 3G-SDI should be enough for at least 10 bit RGB (not sure about the frame rate, but I think it was 2K p30 with RGB12 that was too much for 3G but 24/25 worked afair). A fast Google search brought me this:
https://wolfcrow.com/blog/what-is-6g-sdi/