It's a bit unclear. Between the cartridge port and the A/V connector you should have access to the signals you need to make it work. If I had to guess, the fact that CSYNC reverses direction is kind of awkward for a device that needs to act as a transparent pass-through for non-32X games.
Without knowing the details, I'd guess that in order to link the VDP, you'd need to connect via pins that weren't available on the cartridge slot or the expansion slot. It would have to have been planned (or at least considered) during the design of the genesis/megadrive.
