There’s no signing mechanism (@distopia), but there is a mechanism to defang the outer tar format layer of the Qubes OS backup file during restore:
Assuming that the backup file is being provided by a VM (i.e. it wasn’t manually copied into dom0!), restore.py will not deal with complex tar format data directly. Rather the VM has to convert it to the simpler qfile format (as used for inter-VM filecopy, or for receiving dom0 .rpm package updates) before streaming that to dom0, where the stream will be processed by qfile-dom0-unpacker instead of the tar utility.