I've seen a few on cars here and there. Mostly to keep the back window clean. Problem is in a stationary home heating system the turning vane just puts pressure on it's mount as the air pushes on it. Mounted to a car it would tend to induce drag as it pushes through the air. Now weather or not the improved flow cancels out the induced drag probably depends on the arrangement. I'm guessing it would probably be more useful if it reattached flow somewhere up front.
An example of a pickup turning vane
