It shouldn't have to forbid that. The editor and privileged files shouldn't be in the same string. It should just be appending the list of temporary files to the editor command, and running that unprivileged.
You don't even need a temporary file; opening the file directly in sudoedit then passing /dev/fd/N to the spawned edit process after dropping privileges would work (a-la capabilities). But sudoedit being implemented in terms of sudo makes it hard.
edit: apparently things are more complex and sudoedit already runs the command unprivileged; the attack is in filename expansion in sudoedit itself.
Besides the fact that that wouldn't actually have prevented this bug (which you acknowledge in your edit), /dev/fd/N is a linuxism, so wouldn't work on other unices. And has slightly different semantics than the current implementation, where your changes to the file aren't actually updated in the original privileged file until after you exit the editor.
