What, put a 2S balance plug on the main power of the battery connector? That should work... as long as you don't need it for other devices. Wouldn't it be easier to just solder a 2S balance plug to the existing 3S charge wires and make it like a "Y" servo extension, keeping both plugs? A little heat shrink on two wires; easy-peasy.
The battery arrangement was, I'm sure, designed to satisfy the current "State of the Art"; which is "rechargeable by just plugging the TX into a wall-wart charger overnight". Also, it's probably the cheapest SAFE (Read 1D10t-resistant) way to include a rechargeable battery with the radio. There are a lot of users who are going to just use it that way.
I've flown my bare frame a couple times with the KK2; last time on the bench I was still trying to get my resonant vibration within a reasonable level. Have hung it on the wall for now; been a bit busy with work on my car and watching the kiddles now that summer is here. And lots of wind still playing havoc with me. And it is already CRAZY hot over here; not a lot of fun being outside for ANYTHING.
mnem
*sizzle*