>>It'd be nice to search only in message headers for text matches. Then when I do something like search for 'suggestion' in the chatter category, I only get the messages with that word in the header.
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>Indeed, and/or to limit the results to the first header/posting containing the search phrase - you can always expand the thread if necessary.

Here, here!

Actually, I think your suggestion is more useful than mine. It's implementation is independent of mine too.