So, https://twitter.com/sunjammer/status/569925239989256192 and https://twitter.com/sunjammer/status/569983534368206848
Now, depends on whether you want custom BRDF in deferred on 1) all your objects (easy) or 2) only some of your objects (harder).
That's fairly easy. The actual object shaders more or less just fill the g-buffer (and in Unity 4.x deferred lighting, combine textures with the lighting buffer in the final pass). The actual BRDF is in the "light shader". Start by copying the built-in light shader (4.x deferred lighting: Internal-PrePassLighting.shader; 5.0 deferred shading: Internal-DeferredShading.shader) into your project. In 4.x, put it into Resources folder so it's included into the build; in 5.0 point Unity to use that shader under Project Settings -> Graphics.
Change the shader to do different BRDF.