Well, it is quite possible Manning could generate the largest birther rally ever.
First, he'd have to top the massive crowds that showed up to the steps of the Supreme Court in December, '08 in a show of support of "citizen attorney" Leo Donofrio:
After that, there was Orly Taitz and her legion of followers crowding New York's Times Square in a November '09 attempt to shut down Fox News (If you squint, you can sorta see them in a small, fenced-in area):
So Manning only needs maybe 16 people and I think he'd hold the record for largest demonstration ever demanding proof of Obama's eligibility. There might even be enough Condi Rice masks!