Poaching is very illegal, yes. It is also very easy to get away with in an urban environment. Game wardens in WA state actually do a pretty good job checkin up on folks. My family alone has had to deal with them twice, but that is only in rural settings, they just don't exist in Seattle. I have gotten rabbits all sorts of ways, including using a longbow. Walking home with two friends, a skinned and gutted rabbit, a longbow, a hatchet, and an enormous cleaver in the very early morning didn't give us grief, and since we got away with that, I just can't get too worried any more.
My urban poaching refinements now include not carrying any obviously fatal weapons. I think I would have a hard time explaining away a crossbow. Traps are what I want to move to, I feel like they would be easy enough to number and build them to make them look official. A friend uses a "hoola hoop of harm" for geese. I want to give that a try as well. It's just a hoop with a net over it to throw like a frisbee over the geese.
I guess what I have really come to realize is that people in the city do not ever expect other folks are trying to kill and eat that bunny they see in the park. They wouldn't consider it. If it is at all disguised they won't catch on. Traps I would check at night, and I definately only hunt at night. I've probably only gone 10 times, but I have never had any problem whatsoever. I also bet that the parks and recreation shirt would work wonders, unless somebody that actually works at the park saw you. Exellent idea.