I gave it some thought and I cannot think of any way to do it
The data is just not available
It may be possible to give an indication of which boards have new unread posts but thats about it
I have created a tool that will show the online users and which boards have posts that the visitor has not read
The settings are at the bottom of the translations file under // ONLINE AND UNREAD BOARDS WIDGET online_unread= Turn the feature on (1) or off (0) title_new_posts = Title for new posts panel forum_path = Path to use in new posts links default is /forum show_forum_paths = Start paths with ./ ie: ./forum or ./folder/index Separate with spaces
Copy to /xtforum_template.css This css controls the output of the [code] BBcode tag This file must exist to turn off the default forum CSS It can be used to hold any extra CSS classes you require for your forum