I was able to run Big Sur Beta in Parallels VM in the following way:
Create USB install media: createinstallmedia --volume <path to usb>
Create New VM from USB drive and customize before install: set Boot flags as suggested above (devices.machwmodel AND devices.smbios.board_id). Disable Parallels Tools autoinstall.
Complete the install process
e.smirnov
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Thank you michis_0806, you workaround works for my T2-enabled MacBook Pro.