The final chapter has been written, at least for this little experiment. I crammed two of the 9.6V NiMh battery packs into the tender. I didn't have room for the charging jack I had installed, so I moved it to the underside of the tender. The locomotive and tender are connected using a 2-pin 2.54mm Molex connector set, and I used the same style connectors in the 3-pin format for the tender internal wiring. The seemingly "extra" connector set was do I can separate the tender shell from the frame, I abhor installations where shells are hard-wired to the frame! So, I present the "finished" product. The odd hole in the frame is where my charging jack used to be, but there wasn't space for it with the two batteries.
Since there are two battery packs and only one charger, I just split the charging and will charge them one at a time when needed. I wasn't 100% sure about charging the two in series, and I don't have a charger that would do it anyway.