I am not sure if this is helpful, perhaps you have explored this option, but have you tried leanpub??
Its kinda agile publishing; Gojko Adzic used it when publishing one of his books, those that signed up as interested got regular updates to the book and were i think able to offer feedback. also he has been able to supply additional material.
For his books, he published as pdf, mobi and epub; which was useful as i could have a native edition for my kindle, and use the PDF on my mac and iThings.
Anyway just a thought
Very good thought. I heard of them but hadn't looked into it (wasn't an option back when I tried). I may take another look but…
This thing that Lulu gets me, and it's not clear whether leanpub does this (looks like it doesn't) is transparent support for iBooks and BN.com (and iBooks does free updates). And no matter what I do, Kindle has to be dealt with separately.