I wanna say that was an option with one of the driver disks for one of the GD5428s I had back in the day... that was 2 moves and one continental relocation ago though, so I doubt I still have that one. I wanna say it was a card by Pine, VLB, but it could have been expertcolor or noname whitebox.

12bit results in a huge performance loss as every other pixel is shared in memory with another, and it takes many extra instructions to do any of the rendering when these pixels are manipulated. Video performance is likely to be several times slower due to many more CPU cycles being burned in handling the pixels which is probably why 12bit never caught on.

Even if hardware blitter exists on such a card, it will similarly be limited in performance as it has to do read/modify/write cycles a lot which are a lot slower that raw reads and raw writes.