How is "auto-success" a metagame mechanic? In that case every spell in D&D is a metagame mechanic, given that they are always successfully cast (eg no misspeakings, magical vortices that muck them up, fumbling with the components pouch, etc).
I thought a metagame mechanic is something that generates a change in the fiction, or perhaps the resolution process, that doesn't correspond to anything the PC does. When a player in Prince Valiant spends a storyteller certificate, his/her PC is doing something - in that example, is looking for something.