Idk, I tried a few times... literally just open RMT, go to the CO page, set it to -20, reboot: "blue screen" during startup saying something about Windows cannot verify this file and needs to be recovered. Not sure if it was an actual BSOD, or a message designed to look like one. Cleared CMOS, booted fine. Probably only really needed to reset CO in the bios but I hadn't found where it was hiding at the time
Repeat with -10; same outcome. -5; same outcome. Finally found the bios settings, set -20; booted just fine. Didn't pass stability testing, but at least now behaves like I expect an undervolted CPU to behave rather than a total block during the boot sequence.
It's not important, of course, but it is odd to me that RMT makes Windows 11 so very unhappy. But I'll just be leaving it alone now, this new build is so much cooler, quieter and faster than my 8700K that I'd really like to stop messing around tuning it and put it under my desk properly