I don't like this idea... It adds too much extra to the page... A suggestion that is floating around is that the avatar's become faded or even grey scaled when the thread it read... However I am almost sure you would be able to add this to the template system manually.
But it is only an additional indicator, the thread title is bold/un-bold as standard so doesn't really need additional identifiers but the profile pic being faded/greyscaled or anything else would add just that for you...
Personally I find the vb statusicon system to be a less than useful bit of clutter. Most folks have no clue what they mean, and making site specific icons is a nightmare for graphic designers when there are sooooo many different values .
Almost everyone on the internet is familiar with the darkened/non-bold text indicating something old that they have seen, I tend to think that relying on that existing schema (in the psychology sense of the word schema, rather than the DB sense) is a better bet. It also has the side effect of being cleaner, less cluttered and producing faster loading pages .
Obviously YMMV widely though, so as long as it's easily turned off whatever floats your boat is cool.