This is in fact how we export spherical maps. Except we don't play with tons of light sources. You can just pipe all your shaders through a surface layers luminosity set to 1. Turn off all suns, environment light, GISD, and render like you did before.
You can also do the same thing with a Displacement to Scalar shader into the Luminosity and export a 32bit EXR of the planet as a heightmap.