I'd be swearing out loud.
My guess, and only a guess, is that something like this would be a little dangerous to do. It being a database, its important to replicate/synchronize full database objects (tables) and not partial database objects (rows/columns).
Depending on the database design, i would imagine that what we are asking there, would mean this request would be asking for update of just rows or columns, because individual topics are likely rows within a "topic" table.
Now again, a lot of this depends how LW does the database updates/syncs, and maybe they do a whole bunch of update/insert statements when we click the sync button (but i imagine not), in which case such a thing could be possible.
Dont get me wrong, I'd love to have an individual topic sync/publish, so if it is doable, I'd be definitely voting "+1".