Tiled edges should not take any time to build

When generating a non-square terrain you set it to only build certain tiles in your tiled build but whenever you have those empty tiled edges… say on a 2x4 map. It still builds that whole 4x4 and while I don’t mind doing the extra operation to crop the image, the tiles are actually built making the texture build take near 2x as long and 2x as much memory. Is there anything that can be done about it?