Wow. I don't know of anything that creates rotatable textures automatically. I imagine you can do it by hand but it'll likely be a bit of a pain. Make two edges seamless (or use a generator) and then copy those two edges to the other sides and blend them in. Pattern avoidance will be tough, however, and I suspect tiling will be pretty apparent unless you're really careful. I suppose this'd rightfully be called a geomorph generator...and if someone made one I'd buy it for sure. What a valuable tool for gaming.
M