Hi Crash,
You can paint directly on the mesh but the UVs will still need to be completed. The painting is referencing the texture space and it’s location in the 0,1 space. Even using a Flatmapping would be feasible to break the polygons and lay them out to be painted. The problem with that is when you have a more complex model it will break more apart and generate something that has significantly smaller polys. Again, for the painting this won’t be bad. When you create the second UV channel for lightmaps and if you leave it like this it will greatly affect how good your lightmaps appear without having to jump the lightmap resolution up to some significant size.
Tim