in the case you only want to categorize replies in one topic, so that people find them easily; a simple trick would be to use hashtag+word as the and the “search inside the topic” option .
e.g.:
when a reply is in the category “about the author”, I need to tag it as #about the author, then if one uses the search option he will get something like this:
of course you can make a simple drop down filter so that it’d be easy for people to do the search.