I would recommend any of the walkway tunnels in either Prospect Park in Brooklyn, N.Y. or Central Park in Manhattan. They range for 10 to 20 yards in length and have nice rounded ceilings. It is not unlikely that the NYC Parks Department would grant a permit for a nominal (or no) fee if use were to be more than casual (hour or more?) I often play pennywhistle in these venues, and the accoustics are great. The underground walkway between the Chambers Street subway stop and the World Trade Center also has great echo. Pedestrian Traffic at night is not massive. (An added bonus, for music lovers, is that you get to "Take the 'A' Train" to get there --no kidding.)

Happy Hunting,

Tom D.