Rick, interesting. I mostly use the Duracell Pro Cell AA's, and my experience with consumer-level rechargeables has been less than appealing. Just to make sure I'm not misunderstanding, when you say "Endloops" you're talking about Sanyo brand Eneloop rechargeable batteries? I've never used them but maybe I should give them a try. While price of batteries wasn't really driving my decision, you make a great point.
I wish Sony made a kit like the Sennheiser has. On the Sonys, both the transmitter and receiver have a little AA battery cartridge that clicks in there. Even if I used one of the AA battery eliminator kits that others have suggested, I would still have to make a modification to that battery cartridge to allow the wire to come through.