Fragment Shader Normals in DepthNormalsTexture

I'm wondering how I can go about storing the normals from a fragment shader so that they are accessible when rendering a DepthNormals Texture. The normals from the fragment shader are derived from a normal map. The surface shaders in Unity handle this wonderfully which can be demonstrated by placing a simple mesh with a material with the diffuse normal map shader, in front of a camera and rendering the depthnormals texture. The normals from the normal map are accounted for when rendering the depthnormals texture and accessing the normals component. All I want to do is have the same functionality of accounting for the normals of a normal map, but for a custom fragment shader that also uses normal maps. Has anyone done this before or does anyone know how Unity does this internally?

 

If there's not a way to get the normals from a custom fragment shader into a DepthNormalsTexture then can someone let me know if I just need to move on? There surely has to be an alternative though, right?

 

Bump. I tried checking compiled surface shaders and the compiled depthtexturenormals replacement shader and found nothing that seemed to be internally storing the normals in the surf shader for the depthtexutre shader.