It seems silly that I can’t bail out of the fragment program if I’m processing a fragment that was rasterized from a back facing polygon.
I question your intent.
Backfacing polygons are normally culled by backface culling. That is, they don’t get rasterized anyway.
However, you can rasterize back-facing polygons if you disable backface culling.
But then, why would want to allow backfacing polygons, and then kill all the fragments there? Just enable backface culling and be done with it.