I recently moved to Sussex County and have not found a place to ride at night. I've been asking around but have gotten the usual "...parks close at night..." response.
I used to ride at Blue Mountain in Westchester (covertly) and occasionally, the MTA Lands in Brewster, but both too far now.
Brewster was great because the locals marked the trails with reflective paint, and held weekly rides on either Tuesday or Wednesday nights. There was always a big trun out and it reminded me of the opening scene from ET.
It's been a few years since I've been back to Brewster, so I don't know if these rides still go on. You may want to troll the forums at
http://www.wmba.org/ and post up there.