I am going to try now this in Terminal:
/Applications/Install\ macOS\ Big\ Sur\ Beta.app/Contents/Resources/startosinstall --DoNotSeal TRUE
Crossing finger that this works.
Epic fail:
unrecognized option `--DoNotSeal'
Usage: startosinstall --volume <target volume path>
Arguments
--license, prints the user license agreement only.
--agreetolicense, agree to the license you printed with --license.
--rebootdelay, how long to delay the reboot at the end of preparing. This delay is in seconds and has a maximum of 300 (5 minutes).
--pidtosignal, Specify a PID to which to send SIGUSR1 upon completion of the prepare phase. To bypass "rebootdelay" send SIGUSR1 back to startosinstall.
--installpackage, the path of a package (built with productbuild(1)) to install after the OS installation is complete; this option can be specified multiple times.
--eraseinstall, (Requires APFS) Erase all volumes and install to a new one. Optionally specify the name of the new volume with --newvolumename.
--newvolumename, the name of the volume to be created with --eraseinstall.
--preservecontainer, preserves other volumes in your APFS container when using --eraseinstall.
--forcequitapps, on restart applications are forcefully quit. This is the default if no users are logged in.
--usage, prints this message.
--volume, path to the target volume.
--user, an admin user to authorize installation.
--passprompt, collect a password for authorization with an interactive prompt.
--stdinpass, collect a password from stdin without interaction.
Example: startosinstall --volume /Volumes/Target