Ok, I don't know the area or any relevant laws *at all*, so this may not be helpful at all.

That said, it looks like if you can navigate to Riverside Drive (via 181st street?) there's a footbridge and multiuse path north of the George Washington Bridge that will take you over and then under the Henry Hudson Parkway and connects up with the Greenway path.

http://maps.google.com/maps?f=q&sour...,0.003259&z=19

Edit: It looks like there's something similar south of the GW Bridge off of Haven Ave near W 176th, but it's much less clear where exactly it goes.
http://maps.google.com/maps?f=q&sour...,0.003259&z=19