I might be giving up... I tried all three bins without success. I disabled SATA ports, minus the main ports (all AHCI), disconnected most disks to check for conflicts (all my SSDs and the NVMe are Micron/Crucial), but no dice. Seems like my trusty UD5 Rev 2 doesn't want to play ball. I wouldn't be surprised if it's a user issue but at that stage and two days into making this work I'm at a loss. Thanks tho for your help and responses, appreciate it.
/edit: I got the NVMe to show in the BIOS. I had to go back a few BIOS versions (last non Beta version, FD 2011/02/01), plus the SS950.bin for it to work. This was a heavy birth, but absolutely delighted my trusty x58 board was able to pull out maybe its final trump card.