Take an orthographic camera and point it minus 90 degrees down, and position it so it matches the heightfield you want to output at the borders. Then render that. As baking is technically a render anyway IMHO, it's essentially the same thing, but with a manual bit inbetween that won't cause anyone to break into a sweat. You should also switch off the Atmosphere in the planet to get a true RGB render of the terrain colours. If you want to get an albedo colours render so you can light it in-game instead, copy the diffuse colours to the Luminous colours of the Surface Shaders, and switch off any lights..... theoretically.