I've just spoken to Jeff Jones about innertubes for the Endo – he uses 2.1" Maxxis superlights !!!! They weigh 99g – he says it works. He casually mentioned he'd had one in there for 3 months before it exploded – so not an ideal solution! But a great way to loose nearly a pound in weight.
Do you use rimtape? I thought you wouldn't need it – as the pressure is so low that the tube does not touch the cutouts in the rim, or the spoke nipples – forgot to ask Jeff about it. My LBS said I'd need one and gave me an old 20" tube which I've cut down and it fits well.