This is probably too unwieldy but, could you somehow allow a user selected flag to be set that will notify the user on login that a response to the selected post has been made? Perhaps a link on the main page that jumps to the reply.
Something like---
Your post, "Blah, blah, blah", number 9999999<-(link) has a reply.
There are occasions that I would like to be able to see the answer to a burning question ASAP.
please provide instant gratification. :-)