I would actually precalculate the needed images on load of the tile map and then store them. That way you only need your base tex and the masks stored on the disk and the rest will be generated dynamically once for each map.
The reason is that the images etc are no static mesh so multi texturing is nice but a quite overnice setup overhead as well.
The reason is that the images etc are no static mesh so multi texturing is nice but a quite overnice setup overhead as well.