Just wondering, would it be possible to add an option to search though posts in a specific thread? I'd find it extremely useful.

Assuming you're using the new theme, the search box at the bottom left will do it (it may in fact be in the same place in the old theme but I haven't confirmed). If you are looking at a thread/topic, using the search box will search that thread. If you are looking at a subforum, it will search the subforum.