I've been working in a Harmony-based cart dumper. I've been using a Video Game Brain to help. It wasn't working, so I decided to open up my Game Brain and I was horrified at what I found - the Game Brain switches power
to carts and not chip selects! This is a horrible design, because when a cart is not selected, voltage is running through address and data lines while the power pin is floating. Chips are not designed to have power running through data and address pins when the chip itself is unpowered and it can result in damage to games, the system, or both. This explains why my Game Brain rendered my Kroko cart non-functional for a while. It started working again, but that experience was scary. I don't recommend anyone uses a Game Brain for playing games unless modified.
That said, it is possible to modify the Game Brain to work in a responsible way that will not damage any games. The Game Brain has DPDT switches. This means you can run GND or A12 from the 2600 to the appropriate pin on the cart port. To do it, you need nothing more than wire, solder, a soldering iron, and something to cut traces.
I haven't finished with the cart dumper yet. I will post it to the Harmony forum when something is going. For now, I have a proof of concept going - it operates kind of like the CopyCart.
Here are some pics of how the PCB was wired before the mod. NOTE: the fat traces are NOT grounds! This came as a surprise to me.
The yellow wire on the middle terminal of the switch was the +5v power connection. This wire was removed, as was the unsheathed wire through all of the middle terminals of all switches.
Here are some pics of the completed mod, with A12/GND switched with each DPDT switch: