I am using this tileset in a commercial game and plan to ship a level editor, but allowing for full customization of the level background would be redistribution as you've defined. Having the user import a resource wouldn't quite work either, as the game relies on a modified version of the tileset.
Any ideas on how I can approach this? Worst case scenario I can just remove background customization from the editor, but I would definitely prefer to allow it somehow. I would be willing to explore licensing options, e.g. giving you a cut of sales or paying up front.