Why not split this into sub modules? Like letting Rails have its own portion so that you can add in this like Sinatra, Nokogiri and the likes?
I don't fully understand... are you talking about Git submodules? Or just allowing to generate the docs for any Ruby project? The latter is already high up on the feature list... :)
(feel free to reply in the Disqus comments on ruby-docs.org, I don't check the comments on HN very often)