Hi Mike,
I don't currently have a solution to recommend for this. I will ponder on it for the future. However I would think something like that needs to be obvious to the user what is being searched or filtered out otherwise it may filter out search results that the user was actually looking for. Unless your pages have duplicate content or the user is searching on very generic terms there should not be so many search results that the user cannot find what they are looking for. While I can understand a section author wanting to see only his own section when searching I'm not sure end users really benefit from that unless there is an obvious mechanism for it like choosing a section from a drop down (which would be a fairly big thing to implement).
One could implement folder based multiple sites with related sites mode so the users and roles are shared, but each site would be like its own section and site search is separate for each site. By using common skins in the sites it should seem all like one site to the user if the sites are linked together in a top menu.
It is also possible to make the search help more prominent so that users can more easily figure out how to refine their search. If you click the help icon on the search rersults page for this site you'll see how to do more advanced search.
Best,
Joe