This sounds great. I'd be interested if for no reason just to support the project spirit. Some thoughts:

1) I'm ok with either single or dual battery. I like the idea of 2 batteries in the same plane instead of stacked, to keep case thickness down.

2) I was a proponent of full sized SDXC but I think it's not worth it any more. 32gb microsdhc is dirt cheap and 64gb is affordable. Full size exists in 128gb but is it worth it? Dual micro slots is probably better than 1 full size if it comes to that. (But probably not feasible).

3) I do very much like the idea of 2mm charge port and hot swapping.

4) It's not worth messing with 18650's instead of flat cells (which aren't that expensive). Flat cells of high capacity are available. In fact it might be nice to have mounting option for add-on layer holding a 4000 mah flat cell like the one in the Open Pandora palmtop. It could be set up with a 5v step-up converter so it would charge the phone like an external charger, rather than going directly to battery contacts. But it would still be a solid piece, no external wires etc.

Please stay away from 14500's, they combine disadvantages of almost every format. One thing I've wanted for ages is a phone powered by commodity AA NiMH (not lithium) cells. Yes they're less efficient but they're cheap and easily available. Good AA's are about 1.2V 2500 mAH so two AA's have about 1.4x the energy of a BL-4J, I think. It could be possible for the external layer to use more AA's, maybe even 6 or so of them. I still have a 6AA holder for my ancient Motorola analog flip phone and it was a practical device.

5) I like the idea of threaded hole over the camera (27mm? 37mm?) to allow screw-in lens accessories available for digicams/camcorders (telephoto/wide angle converter, filters, etc). Might be interesting to also supply some kind of remote flash trigger.

6) One thing I wonder, will the metal case interfere with GPS reception? Same for Bluetooth, wifi, etc.?

Anyway, best wishes for the project.