Hi Apple tree
Go for it! There's a history of guerrilla gardening in New York, and the fight continues to save established community gardens, but there's not a lot of fresh stuff going on and I've seen huge potential in East New York and Bed-Sty if not on the densely populated and fairly well manicured island. If you don't have patches in mind and can't find any perhaps there is a community garden nearby which you can help in, all be it not as a guerrilla, but hey, you'll still be gardening and doing it with some new folk.
Richard