Hi Markus
I started work on Inkscape earlier this year (April/May). During the summer I work with Commons Machinery [1] to improve metadata handling and the GUI for metadata and license information. In this prototype I'm working with the idea that multiple metadata tags are allowed and therefore multiple rdf tags (though only one per metadata tag). With my limited experience with Inkscape codebase and the object tree I'm not sure how to best refactor RDF to fit in.
In an earlier discussion with Peter Liljenberg [2] he said that a refactor of RDF should be done to be more general and easy to work with. I'm not sure about the specifics about this but I will look into it later on in August hopefully.
You seem to have a lot more experience with the Inkscape codebase than I have so it would been fun to discuss any changes that is done here in this area with you. The goal now for me is to complete the prototype with as simple code as possible and then during the summer/fall (when time permits) refactor so it can be merged into Inkscape trunk.
Concerning GSOC merge before 0.49, I don't know.
[1]
http://commonsmachinery.se/[2]
https://launchpad.net/~peter-liljenberg