Update 8/14/13: Follow up by members of the WSP Hawk watching community has solved part of this riddle. The band NJ 30 was placed on the bird at Teterboro Airport, Tererboro NJ on July 13th as part of a Bird Air Strike Research project.
The bird returned north rather than continuing south after being relocated Bobby and Rosie both socialized with the bird Bobby allowed the bird to take a rat from him last Saturday The plumage and markings are consistent with past fledgling photographs, although there are no distinct field markings that make it a sure match.
I'll leave it up to the reader to decide if we have enough evidence to declare NJ 30 a Washington Square fledgling and not a vagrant first year hawk passing through the park. I know I have too much of a bias to make a clear call!