I have been able to update from Monterey Beta 9 to RC2 in a MacPro 4,1->5,1 thanks to the user
@cdf modifying the config.plist included in Martin Lo's package, setting VMM to "OFF", changing SecureBootModel to "Default" but also changing the
FirmwareFeatures . In Martin's config.plist the PlatformInfo>PlatformNVRAM is not included, so I had to include it manually and set both the FirmwareFeatures and FirmwareFeatureMask as explained in the FirmwareFeatures tutorial included.
I include here my own config.plist to help those with MacPro 4,1->5,1 and 5,1 users to update.