Thanks Walter, worked better than my solution.
After I posted my question, I ended up getting to a very similar solution, but I used the B&W effect instead of shift channels.
I added another layer of the original source material (with the green patch) above the keyed in layer, but below the source layer. I applied a B&W effect to the intermediate layer, then did a multiply blend on that layer.
B&W resulted in a darker image though.