Well... If you went with a more painterly style, it would not be "quite" as hard to match up and you could use something like Posterazor to assemble. So, perhaps 2x2 3 long and 2 wide images. Even then, the images would be 7200x7200 px. Another alternative would be a hybrid application such as Xara which might be able to deal with the size since it's really only "set" at export time. The issue with that would be making sure your source textures were of the appropriate scale when you can't directly "see" them.