Since this probably will not be added any time soon, try using Watchlist
https://bitcointalk.org/index.php?action=watchlist
Why not? You click watchlist, you see someone has posted a reply (the casually monitor part) and can decide if you want to read it now (click on it) or not (do not click on it). If you read the new replys you can still decide if you want to answer or not. And even if no new replies came in since you last checked and now want to answer you can still find the thread under Watchlist -> Edit Watchlist.
I dont see how going to the subboard in question and searching the x first pages for the thread is better.
Also OP asked about topics started, which are automatically on your watchlist and have to be manually removed.