Similar to the question here, is there a way to change the login background wallpaper in Big Sur? During the beta, I was able to do it via a combination of the procedures in Catalina and the bless command described at https://eclecticlight.co/2020/06/25/big-surs-signed-system-volume-added-security-protection/
This doesn't seem to work with 11.0.1 (20B29). It keeps reverting to the com.apple.os.update-779B* snapshot. Note that I am using FileVault.