I don't think the actual HN "database" has that much info. You would need to reconstruct this from the complete server logs, along with application logs for PG's direct updates (assuming he doesn't use the web interface)
It has to at least know who upvoted, to prevent revotes.
HN has to know who voted, not which way someone voted. HN doesn't let you change your vote once its been made, but it does let you downvote once you have enough karma.
I don't think this would work well. The occasional false-positives would look like annoying glitches, every now and then making it look like your account had been hijacked by hiding the buttons as if you had already 'voted'. Although maybe I'm not getting the math right?
Also, the existence of the 'saved stories' link from the user page would seem to indicate that for stories at least a full list is being stored.
