That sounds pretty slick!
It would absolutely have to have scripted signaling so people using CouchPotato, SickBeard, torrents or any of the other automated downloaders could function.
It would also need to be hooked into the Mover somehow so it can do its job without bringing the whole system down.
I think the password protected 'Please' method would work well for scripted activities from desktop machines. For example, I run SyncBack to do backups to the server - I can have it execute a batch file to copy/create the key file before the backup starts, then another (or, gasp! the same one with parameters!) to remove it when the backup finishes.
Nothing Squid likes more than feature creep!