Now that the new tile set is done I have an idea for a much simpler solution (actually I had this idea a long time ago, but didn't pursue it further). We could assemble all walls out of 3 separate meshes: A wall, a corner and the roof. The wall and corner parts would get rotated like the old tiles and get placed beside the roof parts. The problem is, there would be probably some overdraw between the parts and the walls would reach into the adjacent fields. However, I think this new solution would be more flexible in regards of wall variations and graphically more appealing.
I've sculpted a first mock-up version of claimed walls which are not tilable yet. I'm not sure if that would work for the dirt walls as well.