I came here through Robin Good's news, following a string in the George Siemens digest.

You are making a good point about context being separate from content, and about context usually being user-created. That brings to mind a platform that I recently heard of, which allows just that.

Shift Space

http://shiftspace.org/what-is-shiftspace

seems to be doing exactly that.

"ShiftSpace is an open source layer above any website. It seeks to expand the creative possibilities currently provided through the web. ShiftSpace provides tools for artists, designers, architects, activists, developers, students, researchers, and hobbyists to create online contexts built in and on top of websites."