>From the Plane of Air, a spell that lets the caster
>actually become mist-like when confronted by a door or
>gate.
>
>CF Magical Dogma rates that as Transmutation. Invokers
>create, control, or destroy elements.

On the other hand, an invoker spell could surround the invoker with a field/shield/etc. which makes all non-moving stone or metal (or something to that effect) turn non-corporeal for a few seconds when it touches.

That said, I'm not overly fond of this idea, considering it would weaken invokers by making a certain element necessary to pass door.