Using Texture2D.ReadPixels with a RenderTexture that has AntialiasingAsPostEffect

I am trying to save .png's from a RenderTexture. For my purposes, I need antialiasing so I've got the AntialiasingAsPostEffect on the Camera that the RenderTexture is attached to. I have the render texture showing on a Plane in my Unity scene just so I can keep an eye on what's being rendered to it. The PostEffect AA looks good so it seems like I'm ready to go, but I find that when I set RenderTexture.active to that render texture, the files I save are blank.

Here is what my code is like on a script on the Camera with the RenderTexture:

 void Start () {
     camera = transform.camera;
     renderTexture = camera.targetTexture;
 }

 void OnPostRender() {
     RenderTexture.active = renderTexture;
     Texture2D frame = new Texture2D( renderTexture.width, renderTexture.height );
     frame.ReadPixels( new Rect( 0, 0, renderTexture.width, renderTexture.height ), 0, 0, false );
     frame.Apply();
 
     string fileName = "frame_" + framesCaptured.ToString( "000" ) + ".png";
     string workingDir;
     if ( Application.isEditor )
         workingDir = System.IO.Directory.GetParent( Application.dataPath ).ToString();
     else
         workingDir = System.IO.Directory.GetParent( Application.dataPath ).Parent.ToString();
 
     byte[] bytes = frame.EncodeToPNG();
     FileStream file = File.Open( workingDir + "/Saved/" + fileName, FileMode.Create );
     BinaryWriter binary = new BinaryWriter( file );
     binary.Write( bytes );
     file.Close();
 }

If I comment out the first line of the OnPostRender function and just read from whatever is already the current active RenderTexture, it seems to read from a TemBuffer from the AA PostEffect and it doesn't have the AA applied yet, so its still all the jaggies. If I leave it in, the files I write are empty.

Can anyone help? I need to save files from this RenderTexture without jaggies!