I successfully installed the beta of macOS Big Sur 11.0.1 by spoofing only the board product and changing only a single bit of the firmware features:
XML:
<dict>
<key>BoardProduct</key>
<string>Mac-7BA5B2D9E42DDD94</string>
<key>FirmwareFeatures</key>
<data>A1QM4A==</data>
<key>FirmwareFeaturesMask</key>
<data>P/8f/w==</data>
</dict>
By default, the firmware features of the Mac Pro 5,1 are set to
A1QMwA==
. For some reason, the changed bit fixes the "Your Mac needs a firmware update in order to install to this volume..." issue. (Interestingly, this bit has been documented as the one for enabling proper UEFI bootability for Windows...)
Unfortunately, Lilu userspace patching is broken in Big Sur, so no Night Shift or DRM without more spoofing. If Lilu patching is not fixed, we'll have to seriously consider properly changing model and generating serials to regain proper functionality...