Since Rhinos view port render mode has gotten so much better Im actually using it for final images in many cases. My main issue was getting an alpha for the image without rendering. My work around was a view port render mode I created to do just that. Combined with ShowZbuffer you can get an alpha and a depth map without rendering. If you want alphas for specific objects simply turn the layers off you dont want visible and save the image. Its a few extra steps but your not waiting on a render. Its also useful for creating black and white images of 3d objects for creating photoshop brushes.
Ive uploaded the INI for anyone that wants it.
Alpha channel.ini (11.4 KB)