Fiddling around with the idea of an icon to represent "Unread Messages In Reply to You." If you log in, I've currently got one working next to where it says "Welcome, [user name]."

(I think, I don't know, nobody's actually replied to me yet).

Currently, it shows the "search" icon if there are no messages, and the "reply" icon if there are unread posts in reply to one of your own posts. Clicking on the icon in either case brings you to the search screen which shows you the full results of the "Unread Messages In Reply to You" query.