@CaseySJ Looks like it works. Also, when I dumped the preboot.log with F2, Clover did view the RAM as it is in reality, not SMBIOS. It was as I expected it to be but it never hurts to verify.
I pretended that I had 8 RAM slots, and that 0 1 2 3 mapped to 4 5 6 7. So I have a 16GB in slots 0 and 2 and a 32GB in slots 1 and 3, so I filled in 16GB for 0 1 2 3, and another 16GB for slots 5 and 7, as if the sticks were double wide connected to an imaginary slot at the bottom of each.
I don't know if any of those positioning matters in the slightest, because I was able to fill all that in and boot with one of the 32GB UDIMMs missing and Mac didn't care and still listed 96GB in About This Mac.
So based off this I see no reason that this method with SMBIOS filled out for 8x16GB properly won't work for a full 128GB. At some point in the future I intend to max it out, but at $150 per stick of RAM I'm pacing myself.
I'm wondering if there is any chance that this isn't working like I think and Mac is only using them to half capacity. I doubt it but you never know. Activity Monitor reports 96GB. I think I'll just spoof a full 128GB and see if that falls for it. I know About This Mac trusts whatever the SMBIOS tells it, and I can lie to it there.
For a more visual explanation. For anyone going full 128GB just fill in 4 and 6 with another 16GB stick, changing the frequency accordingly. I doubt the Vendor/Part/Serial matter but may as well change that.
I set 2667 instead of 2666 because on a real iMac 2019 it reports 2667 in About This Mac and I wanted to match that. I guess it just rounds up as I can see it sometimes hit 2667 in the BIOS.
View attachment 424741
EDIT: If I spoof 128GB, 32GB more than I have, it shows 128GB in Activity Monitor and CleanMyMac, so it's a widespread effect. I set it back to 96GB to match what I have. Unsure of how I can test if it's being utilized properly.
Other than that I wonder if this is something Clover can fix automatically in the future. A real iMac19,1 already supports 4x32GB configuration as is.