yeah I used pretty much the methods you said and achieved this but in 3dsmax, so texturing model based levels does work but with some kind of gimmicks.

Had to first paint red,green,blue,black as vertex paint on different channels, then add the corresponding colors as materials to the same channels. Then unwrapp the thing nicely (hate to uwnrapp terrains V_V), and then render to texture diffuse, then the shadowmap, and this is what I got in 3dgs with the shader from Slin & co from the free outdoor weather dynamics+shaders pack.

Pretty nice if u spend some time on the terrain if you ask me since I did it in 5 minutes just to try to actually get it to work in 3dgs a7.