If you are trying to UV the high res for the diffuse you dont need to. Xnormal will bake polypaint to low res uv mapped model. In Xnormal check "Bake highpoly's vertex color" in the baking options. Make sure in the high definition mesh that the "Ignore per-vertex-color" option is unchecked then you will get the poly paint…
OK, I'm in a situation of my own making because of lack of planning. I have a nice highpoly sculpt in Zbrush. It's a dynamesh object so there are no lower detail versions. I would like to make a game rez version of this but there is the issue of UVmapping the high so that I can transfer polypaint info to a low rez. I have…