I fiddled around with the Valet+ driver (which should also fly for Valet) and got the upgrade working on a clean starter kit. I added an old style content folder and watched the script update it.
Just need to add this to the frontControllerPath
method above the existing rule for index.php
but below the panel rule:
if ($uri === '/upgrade.php') {
return $sitePath.'/upgrade.php';
}
Heres the full driver in a gist.