The Zappers are read the same way as the controllers. Set the strobe to 1, then zero, then read the data on bit at a time.
Barker Bill's trick shooting uses Nintendo's kinda 'standard' controller reading routine (getting from both $4016 and $4017, even, though I doubt they are both used in this game)
To the Earth seems to work more like a zapper-only game, it reads $4016 3 times (trashing the results) before saving any of the data that was read.
Something I hadn't considered that might complicate things a bit, is that the strobe is shared for both controller ports. Maybe we can make it handshake somehow, so the PC software can know the difference between controller reads and communication attempts.
It looks like we'll need someone with a devcart to test this stuff out. I don't have one, though, and it seems the wait for the Maxicart will be a little while longer..
The expansion port would be kinda nice to use, though. But a downside is that newest Nintendos (like the top-loader), and probably all clone systems don't have it.
Does anyone have a rough idea of how much it might cost to make little cartridge-type boards that would connect to it, BTW? I have no idea.
Maybe a Nintendo would also need to sit upside down to hold the expansion cart. But I suppose that shouldn't be anything to worry about. (just write good enough software so the user doesn't want to change carts, heheh)