Yeah, that thread.
A larger battery is an interesting thought. I've been putting off replacing mine, as the ones of same size have less capacity than the original Varta one. There seems to be a good deal of space left for a longer battery.
The three-way switch is likely to be a dead end. As far as I can tell, most of the control circuitry is in the reader, not the battery. Hence my advice to only attach something that will effectively act as a single battery.