The metadata that should be added to the articles in the feed includes:
• Mood (text + icon URL)
• Protection level (public / friends / private)
• Specific friend-groups mask, for your own posts
• Userpic keyword, for your own posts
The latter two are important for allowing the user to edit previous posts, and for posting when used with the Atom API.
Someone would of course have to define a new XML namespace with new tags for this data (as for example the iTunes Music Store does in its RSS feed.) But the actual implementation work seems like it would be straightforward, since the LJ API already accesses and returns all of this info.