hrvoje_hr there are more technical solutions that would require less time for somebody familiar.
All the posts could be retrieved via the API and searched offline. You can ask up to 50 posts at a time that way, and a script could quickly get them all. You'd end up with a big JSON dump.
It should also not be too hard to implement a new gambit that will search for posts in a given discussion. It probably won't be the fastest to implement, but should the need come again in the future it could be useful. Only problem is, there's currently no view for post search, so it would have to be created to be user friendly, or the search would have to be performed via the API.