You can't have a non-round minimap as well as minimap rotation, it just doesn't work right. So, if minimap rotation is enabled, it forces a round minimap.
I don't see any reason why you couldn't rotate the border corner textures as well, as long as you can keep track of what angle the minimap is rotated to. Yes, it would mean doing evil matrix math to figure out the translated texture coordinates, but there used to be good examples in GraphLib you can borrow from. (Maybe it even has functions you can use to do it for you; I haven't looked lately.)