That would make the terrain, "custom" which means it now can no longer be accepted into the MA.
Actually, at one end of the scale your correct. But, also incorrect at the other end.
At one end of the scale, there are two reasons why "custom" terrain textures(tile) can't be used.
1) To avoid mass changes like were seen years ago in AKDesert. Where the entire terrain was mostly shades of brown, red and orange.
2) Changing every terrain texture(tile) in the game would make the .res files in the 10+meg size range. That is not acceptable for a main arena terrain download which should be relatively fast.
At the other end of the scale, changing say one group of terrain textures(tiles) would probably be acceptable if they could be used with existing clutter. Even if that clutter needed to be re-arranged. It would depend on the overall impact on the .res file.
I've approached one of the masters of terrain textures, Nuttz, on more than one occasion to make a single set of textures for a couple of terrains with no reply.
I've also thrown out the idea of 4 res files dedicated to the terrain textures. Once each for winter, spring, summer, fall. Then the person creating the terrain selects which tile set to use for it. Heck, it could also be a rotation option for the arenas.