Once I had determined that the Ryobi 18v tools were just looking for an 18v source and not using any authenticity checks to make sure they had a real battery (as most newer tools do), it seemed like there was nothing standing between making this project a reality other than drilling holes, soldering wires, and a bunch of epoxy.
The only really tricky thing was getting the wires connected as the wires coming off the DeWalt 20-18v adapter and the ryobi battery are rather short, but once that was done, I tested everything and it worked, so I used a good amount of epoxy to attached the 20v/female part of the DeWalt adapter to the shell of the gutted Ryobi battery and it worked.