If you purchase the Checkpoint balancer, then you could just put a female 2mm plug on the center
wire of the Reedy battery's balance plug...being sure to properly insulate and/or tuck away the + and - of the balance plug so they do not short on anything.
With this way, you can use the same balancing harness for all your batteries. Your balancer will pick up the + and - readings through the main charging leads.
Also, you could make a small adapter that converts a JST-XH plug to a 2mm female plug...similar to the one towards the bottom of this how-to article:
That way, you could keep the original plug on your pack in case you want to sell it later, etc.