This is fantastic for cheap baked reflections on glass for mobile and VR! Thank you!
Once you do this, you can increase the resolution of the capture manually by adjusting the render target properties and setting Size X to a maximum of 2048. Clear and reassign the texture target to recapture at increased resolution. For me this was necessary as anti-aliasing was not in the capture, so using high res to downres in photoshop created a smooth cubemap.