Writing to RWTexture3D using a pixel shader without using Graphics.Blit

I'm almost certain that Unity 4.6.1.1f1 still doesn't support writing to RWTexture3D or RWTexture2D from a pixel shader without using OnRenderImage(..) and Graphics.Blit

It seems to me like Graphics.SetRandomWriteTarget(..) doesn't work when used from OnPostRender with either Graphics.DrawMeshNow or Graphics.Blit. I can't understand why though.

Here's a code snippet of the script:

 void OnPostRender()
     {       
         Graphics.SetRenderTarget(rtCam);
         Graphics.ClearRandomWriteTargets();
 
         Graphics.SetRandomWriteTarget(1, rtVol);
         Graphics.SetRandomWriteTarget(2, rtPlane);
 
         matWriteTextureOnPS.SetFloat("_volDepth", size);
         matWriteTextureOnPS.SetFloat("_time", Time.timeSinceLevelLoad);
         Graphics.DrawMeshNow(quadMesh, Vector3.zero, Quaternion.identity);
         // Graphics.Blit(src, src, matWriteTextureOnPS); // replace above line with this and try.
 
         Graphics.ClearRandomWriteTargets();
     }
 

matWriteTextureOnPS has a pixel shader that writes to rtVol and rtPlane which are used as textures for objects in my scene. If I move this code to OnRenderImage and use Graphics.Blit instead of DrawMeshNow and make the shader use vert_img as the VS, it works.

Here're the GPA API logs for the three scenarios I tried:

1. OnPostRender with DrawMeshNow

2. OnPostRender with Graphics.Blit

3. OnRenderImage with Graphics.Blit

For 1 and 3, Unity binds the RT and UAVs correctly using OMSetRenderTargetsAndUnorderedAccessViews, but for 1, it resets the bindings using OMSetRenderTargets. As a result, the UAVs are no longer bound. For 2, Unity never binds the UAVs. I don't know why.

Links to similar questions:

1. http://answers.unity3d.com/questions/566106/how-to-write-to-rwtexture3d-in-pixel-shader.html

2. RWTexture writes are not supported in pixel shaders from camera drawing

3. http://forum.unity3d.com/threads/rwtexture2d-shader-in-onpostrender-problems.173436/