from what i can tell (after conducting rigorous testing) each tileset is organized similarly to the pallets given by the map editors starforge and scmdraft. Each tile in a set as a tile ID, whether this is a number, letter, or group of symbols, i dont know. the problem with mixing tilesets (IE, saving a custom brush from another tileset and using it in a new one) is that tiles in one tileset have the same ID as tiles in another tileset, which causes the custom brush loaded to just be the tiles with the ID's in the new tileset. For example, if you use tile 1 from badlands, save it as a custom brush, then load it in ice, it will use tile 1 from ICE, not tile 1 from badlands. This means that custom tiles are saved by ID, not by tile data. If a way could be found that saved the image, then when loading, it adds it with a new ID that isn't the same as any of the tiles already there, then it might be possible to use the tiles in other tilesets.

badlands has the same dirt and cliffs as jungle, so you'd only need to add the dead grass, some doodads, the asphalt and structure stuff. Even better, if you did it with a badlands map open you could sort of create "placables" like the end of asphalt, the edges, then the basic middle tiles. I know it'd be tedious, but you wouldn't need every variation, you'd have to be selective.