Currently, storage migration is explicitly disallowed with the --tunnelled flag. Since in some cases --tunnelled migration is more useful for an end user (no need to open firewall ports), allowing this combination would be useful.
Still relevant, though it may never be a valid combination since work has instead been focused on adding native encryption to qemu's migration channels
Closing, since we generally want to discourage the tunnelled mode now that QEMU has native encryption, and can use multiple parallel connections to increased speed.