I'm trying to think of ways to do this without requiring
changes to board sw. It think it's possible.

One usenet, the hierarchy (for this purpose) goes
server -> groups -> thread in group
Threads can branch, but reference their 'parent', to which
they are replying. This allows the client to build a tree
of the thread and its branches. Each message in a given
group has an individual post number for that group and
server, and a universally unique (across all usenet)
'Message-ID'.

On the image boards (I may be wrong here, I'm not anywhere
as familiar with them), the hierarchy is similar:

server (eg 4chan) -> image board (eg wallpapers) -> thread.
Each message seems to have an individual post number, which
appears to be unique per server rather than per board (?).

Since pages presented show both the first and last few posts
of each thread, it should be possible for a
browser plugin to keep track which messages its seen on
given server, and know from the webpage which threads have
new content (ie, the last post isn't on the 'seen' list.
It could then modify the presented page to hide threads
and posts with no new content.

Any thoughts?

John Galt