Since you're interested in going for this method, I'd also suggest making a similar sort of file for tilesets. For example, if you load tileset.j2t, tileset.j2t.jcs gets loaded too. The #1 thing I can think of which could be stored in such a file, is what would best be called "templates" - I think I discussed this idea with you before. Templates would basically be groups of tiles placed in a special window, which you can select the same way you select tiles from a tileset. You would also be able to place tiles or events in them, just like you do for a normal level. The purpose of templates would be to provide the mapper with an easy way of selecting common groups of tiles. For example, a hill, or a wall, or some sort of eyecandy pattern, or a cluster of food pickups that go well with the tileset.

The nice thing about this feature would be that it would greatly speed up the level making process, and trying it to tilesets in the tileset.j2t.jcs way would mean templates would not have to be remade for every new level. In fact, tileset makers could even supply a pre-made template with their tileset, to give people a head start.