I might be wrong, but the way I understand it, the SGB essentially contains a Game Boy, so hardware inside the cart itself does runs the code, and just provides the SNES with video and audio and gets controls. It probably uses direct CPU access in many places (the "extra" contacts on the left and right of the cartridge) so emulators and hardware alike would need extensive modifications - there's just no way to access the GB ROM from outside the cart.