The crane stocks will fit lipo sticks in the battery compartments
But I haven't seen an RS stock other than the crane that fits the sticks. Like you said, all other stocks advertised to hold CR123s won't fit the lipos

Anyway. This is airsoft, safe to assume everyone has an airsoft crane stock lol

And I get the feeling you guys are arguing the theoretical, this is already proven to work.
He's got a crane stock
He's got a Y adapter
He's got a lipo stick in both of the battery compartments
It works

This is on a PTW with a shortened board in the buffer tube, it's on the shortest position, and yes the cap goes on no problem
https://fbcdn-sphotos-c-a.akamaihd.n...22971465_n.jpg

They're NOT a tight squeeze. Frank's right, this isn't theoretical, I've already been doing it for months with the adapter, but only yesterday did I hard wire two connectors onto my BOLT. I use this setup in my VFC and BOLT M4, both crane stocks fit the batteries fine. Other types of crane stocks, Systema, Echo1, G&G etc generally have bigger battery tubes so it's even easier. This wouldn't really be possible with a G&P crane stock because I'm pretty sure you can't load them from the back. On my setup I do lose the shortest position but with the right buffer tube (the slots cut out for wires) and with neat enough wiring you could accomplish it.
I find that my guns tend to overdraw from these little batteries, which was causing them damage over time. Since I was storing an extra lipo in the other tube anyway I thought "why not both?" I'm only using brand new lipos with this setup, and they will only ever be connected in parallel to ensure they have the same life.

Thats true. For the magpul CTR + 1200mah battery in the buffer tube, you can get it close to all the way, but you won't get a click. While this may aesthetically seem ok, but if you're shouldering your gun while firing and pressing on the stock, particularly rough when hard gaming, then it just becomes a case of squishing the battery and wires. So, basically for safety reasons, means its the first extension setting it is.