We're definitely receptive to this request (to be able to add more books). We've tossed around this, and we're still figuring out if there's a really nice way to do it while avoiding some messy situations. (Requiring for a web service to upload books would definitely be a barrier for users, as would having to download a separate desktop app, and for us to develop and support one for Mac and Windows.)
Hmm. You are saying we can't add content? That is a deal breaker for me.
While I understand the desire to have a book format that includes extra formatting with pictures and the like, excluding the ability to use the content that is readily available limits it to the point of not being useful IMO.
Right now I have Stanza, Bookshelf, BookZ and eReader installed on my Touch, and despite some issues, have gravitated to Stanza.
In the end, access to content is what has made Stanza my go-to book reader.
Easy ability to download content from Project Gutenberg, and ability to get your own content on there (although somewhat painful).
What I would suggest:
1) Support for existing standard formats (at least start with text) in addition to your own format.
2) Publish your format specs.
3) Ability to download content from the web (enables Project Gutenberg in addition to personal content for the more determined).
4) Host the books you have re-formatted on your web site.
This allows it to be on par in terms of functionality with other book readers, but builds in an incentive to use the content you provide.
5) Release tool to help people create content in your format.
This would help take the burden off of you to create content (esp. Copyright free content).
Personally, I'm fine with paying for a reader. Heck, I'm fine with paying for Copyrighted content (eReader). But I'm not fine with one that I can't add content or determine the content.
YMMV.
Good luck.
-john