Klemens Friedl schrieb: 

Use a wiki with an export feature and export it to a standard open
format that you can plug in to HTMLhelp later.
    
MediaWiki allows both basic HTML syntax as well as the special Wiki
syntax; you can even mix them in one document.

MediaWiki extention - PDF export:
http://www.mediawiki.org/wiki/Extension:Pdf_Export
What about MediaWiki publisher? http://mwpublisher.org/Main_Page
We would still need a chm compiler, but writing the documentation could be done using our wiki.
The previews look very good.

Timo