Technically, that's not a secret Trainer. That's a spell recipe vendor. There is 2 recipe vendors in Wizard101 as I know of. One of them is in Mooshu (as you've already mentioned) and there is one in Avalon. In Avalon, I think it's in The Wyrd near Dun Dara (Well, I DEFINITELY know that it's near the entrance to Dun Dara) It's near a boss which I forgot the name to him.