Since your client (newsreader) is what keeps track of this for you, it should be your client (browser or whatever) that does it in this case.

The first solution that comes to my mind is just sticking a link like this:
<a href="#<?php echo time(); ?>">##</a>
At the top of each post. Clicking it would lead nowhere, but it would remain marked as read in your browsser for as long as it was set to remember such things.