You would repost a page by linking to it from your own. As for uploading content, that is outside of the scope of an indexer. The user hosts their data with some hosting service, or independently. I believe signatures are already handled by XHTML which allows you to add XML signatures to documents, but regular HTML is sadly lacking here thought it would not be too hard to extend (or to just use the XML serialisation of HTML5).

I think it is generally bad to implement new features like this on the part of the indexer. It should just keep track of an existing web of documents rather than creating it's own format and walled garden.