The URL https://graph.facebook.com/search?q=%23hashtag&type=post returns JSON for the #hastag search bar with my account. I would try FB as a sample to find out what you get.
ex. "https://graph.facebook.com/search?q=%23fb&type=post" will return all the latest posts with "fb" in them, regardless of whether the hastag is in front.
It seems I get different results when logging in and logging out. I was one of the first people to access a graph search, so your mileage may vary.
Interesting details:
If the search you do has no hashtag content available, it will only return results from the keyword.
It does not seem to highlight the hashtag that was requested in JSON, and the results for requesting multiple hashtags will be And the results are not OR or
Pager information included in returned JSON using Unix timestamps
ex:
"next": "https://graph.facebook.com/search?q=\u002523fb&type=post&limit=25&until=1373055352"
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