[EDIT: solved, see below]
I will try again this evening. I have the basic config.plist with just Board ID spoofing and the BIOS version mods. 11.2.2 worked fine but I am not seeing 11.2.3.
The update shows up on my 5.1 with vanilla OC 0.6.7, however I'm stuck in an install loop.
1. Once the update gets downloaded, the Software Update control panel does it's things (gradually lowering the waiting time from 15 minutes to zero) and then displays a notification about imminent reboot.
2. If I then press "Update Now" within the SU panel, it starts downloading the update again.
3. If I press "Reboot" on the notification, it displays a black screen with Apple logo and a progress bar, after the progress bar goes to about ⅓, it reboots.
4. Once rebooted, the OC bootpicker shows two "Macintosh HD" volumes to boot from. One of them is the actual system (in that case it's only possible to go back to #1 above), so I pick the other one.
5. The other "Macintosh HD" is the update installer, which only progresses to about ⅕ and then reboots, which means I'm back to #4. Tried booting "the other" like 7 times now, makes no difference.
Oddly enough, the original Apple boot selector (I use a dual-ROM GPU, one ROM is Mac flashed and broken in Windows, the other is stock unflashed and works great with OC so I use this one as my daily driver for OS switching convenience) shows a "MacOS installer" instead of "the other Macintosh HD". It does not boot without OC, obviously.
Also, my actual "Macintosh HD" drive has grown a system snapshot, which I didn't see before attempting the upgrade (see pic). I assumed this to be the update volume, however it shows "Mac OS 11.2.2" in Disk Utility, so perhaps it's not the case?
Any ideas on how to proceed with the update to come through?
[EDIT: solved] I solved the problem by:
1. setting the VMM flags in config.plist and rebooting
2. redownloading the update
So - at least for me on a genuine 5.1 with two X5690, if it makes any difference - the hybridization alone, WITHOUT VMM flags set to ON does not allow for seamless updating.