I tried a different way to modify the 15-100 with a JP connector. It's kind of neat, but may it not be so practical for most people to try. It involves cutting and whittling plastic. I thought it would be fun to share anyways ... If anyone is interested, I'll try to do a real write up. Maybe when I do my second remote.
Basically, I cut out the part of the back cover that is over the JP1 holes (as suggested in this thread) , soldered in premounted pins ( two 3x1 sections) , used 4
shorter batteries (1.5v LR1s), and "McGyver"ed in something to bridge across the top of the batteries:

Nothing really special about soldering in the premounted pins. As mentioned by others, you can get them from Digikey.
The trick is fitting the batteries. LR1 batteries (sometimes called type "N")are pretty similar to AAA batteries (1.5 V, 1000 mAhrs), just shorter. They are little larger in diameter than AAA ones (12 mm vs 10.5mm). As is, they will
very tightly squeeze into the battery compartment. To make them fit easily, I whittled and filed away the plastic from the sides of the battery compartment.
To bridge across the top of the batteries and complete the circuit, I could have just soldered in a wire, and called it a day. Since I was goofing around, I actually cut out the bridging piece of plastic with the metal contacts that's embedded in the back cover. (It's the part that the top of the AAA batteries would normally touch.) I whittled the sides of the cut-out to fit into the battery compartment. To keep the bridging piece in place, I braced the piece against the two overhanging plastic guides that are just below the JP1 connector.
The remote powers up just fine. I'll try a Jp1.3 cable (Tommy Tyler built)on it sometime next week.