This totally works, however I found you have to ensure you are using a forward renderer, and force the camera to render prior to running a screen capture.

Something like:

....
    camera.Render();
screenShot.ReadPixels(new Rect(0, 0, resWidth, resHeight), 0, 0);
....

Perhaps this commandline might work? It's the reference DX renderer which MIGHT work without a gpu since it's pure cpu, although I doubt it :)

"-force-d3d9-ref (Windows only) Make the game run using Direct3D's "Reference" software renderer. DirectX SDK has to be installed for this to work. This is mostly useful for building automated test suites, where you want to ensure rendering is exactly the same no matter what's the graphics card."

I don't believe it's possible to run a standalone player in batch mode without automatically disabling the rendering. As per the documentation:

-batchmode
Run the game in "headless" mode. The game will not display anything and won't accept user input. This is mostly useful for running servers of networked games.

Looks like you're going to either have to think up another solution, or run the game normally unfortunately.