Louvers ...
 

Some of you may still remember the metal louvers that were available for many fastback vehicles in the 70's & 80's.

Thinking back on those, would it be possible to redesign them in order to correct airflow off the back of a car?

That would mean that you get the protection from the sun, the "stylish" look of louvers, plus the aerodynamic benefits of an extended Kamm spoiler.

Or have I finally gone crazy ...

You could probably paint a clear kammback with a louver like look, but it be one solid piece. I don't think you could retain the actual louvers and have good aero.

I don't know if they qualify as louvers (since it's only a pair), but the Merkur XR4ti's hatchback spoilers apparently did a decent enough job at cleaning up aero in tandem that Ford patented them. They're kind of like louvers...

OK, maybe that's a stretch.

It's definitely a good question.

The overall effect of the louvers would be like boundary layer turbulators. You could probably curve them down a bit, quite unfashionably, to take advantage of the increase in boundary layer thickness, but I don't think they cause wholesale separation.