My thought was that the best thing to do would be to add a new parameter to the journal item metadata list called "userMd", or something like that; this item would contain some XML which would specify the names and values of one or more metadata items. (The values would be able to be well-formed XML, allowing for more flexibility.) I think this would require a new db table.
I'm writing a tool which, right now, would have to hide additional metadata in comments in events, which is really pretty horrible. This would make things work much more elegantly.
Is there any interest?