Not sure I understand—HN isn't open-source (though an older version is), but you don't need HN source to work on this. Just figure out how to go from an article's url to the year the content first appeared! [1] That's the hard part. Inserting it into the HN title is of course trivial.
1. Or rather, the year it was last significantly updated.
Anybody want to figure out how to do this programmatically?