Many times I’ve wished that I could restrict a search to the thread that I’m currently positioned in. Maybe another item in the “Search by:” list. Seems as though that wouldn’t be difficult to implement.

Have you ever used this to search? Consensus seems to be it is better than the built-in Sig Forum search feature:

http://www.karmanator.com/sig_search.html

It will not do what you are asking (to my knowledge), so I realize I'm not really answering your question. But if it gives you more relevant search results it might help.