We have developed solutions in which you can search in a folder using the advanced search form: http://addons.alfresco.com/addons/alfresco-share-folder-search
It's free to download, so you can see how it's done.
In short: in 4.2.e you can send an extra param rootNode to webscript / slingshot / search repo.
JavaScript Share components/search/search.js
_buildSearchParams. : site={site}&term={term}&tag={tag}&maxResults={maxResults}&sort={sort}&query={query}&repo={repo}&rootNode={rootNode}&pageSize={pageSize}&startIndex={startIndex}
, rootNode qnamePath NodeRef, .