Store additional sprite gfx in cart?

[Liquidream]

It would be nice if people had the option to swap-in additional sprite tiles for use in their rooms.

Ideally, this would not be stored in additional carts (as this makes the game incompatible with the Lexaloffle BBS).

Current thoughts are:

1. Storing extra "pages" of sprite data in the cart's own data portion (though this may be needed for game saves)
2. Storing extra sprite tiles as a single string, then swapping out at run-time (as detailed here)