Right now it's only being designed for toploader, but the only reason for that distinction is the physical design of the new video board, which piggybacks onto the PPU. Its current design is for the positioning of the PPU on the toploader. A toaster model of the mod will be completely possible, they're just focusing efforts on the toploader right now.
Since Lexi was asking about a composite mod, she logically has a toploader (since toasters already have composite)