Search indexing within a thread

In addition to being able to search for the whole threads themselves, it would also be nice to be able to search for items within the thread specifically. With the search system right now, the best I can do is use in:channel, but that would just show all messages from all threads in that channel, mixed up. I have a lot of threads in each channel, and each thread can be quite long, so this search indexing feature would probably be very useful. Something like an in:thread, perhaps?