I took Chris_F's BRDF containing Cook-Torrence/Schlick/GGX/Smith and Oren-Nayar, and re-implemented it with hacked support for anisotropy (based roughly on Ashikhmin-Shirley) and retroreflectivity.
The features that I think I need so far are: Non-lambertian diffuse, IOR/F(0º)/spec-mask, anisotropic roughness, metal/non-metal, retro-reflectiveness and translucency.
If both the roughness factors are equal (or if the isotropic bool is true), then the distribution should be the same as GGX, otherwise it behaves a bit like Ashikhmin-Shirley. Also, the distribution isn't properly normalized any more though when using anisotropic roughness.
The retro-reflectivity is a complete hack and won't be energy conserving. When the retro-reflectivity factor is set to 0.5, you get two specular lobes -- a regular reflected one, and one reflected back at the light source -- without any attempt to split the energy between them. At 0 you just get the regular specular lobe, and at 1 you only get the retro-reflected one.
BRDF Explorer file for anyone interested: http://pastebin.com/6ZpQGgpP
Thanks again for sending me on a weekend BRDF exploration quest, Chris and Promit