tl;dr: bake the damn things.
After painting, from the paint room go to Textures > Texture Baking Tool
Fill in the stuff on the dialogue box as appropriate (I have no idea how correct my settings are, I tend to take a best guess with anything I don’t know :P)
Export the model as a .dae, import into Blender. It seems to import as many Ptex materials as layers you used in the 3d Coat’s paint room.
