You might want to try directly contacting the C9 developers via this link:

In practice however you'll probably find that C9 has a record of which videos you've voted for, rather than which videos you've watched. The links that change colour / boldness once you've visited them appear to be remembered by your computer via a cookie rather than C9's servers, so you might find that this isn't a question that can be trivially answered by the C9 team.

Still, there's no harm in asking again.