I have to say that while it ~could~ be done, it is a really bad idea. 1. you would be trespassing regardless if there is a sign saying such or not. 2. because there is a fence, you would also be breaking/entering into a construction site. 3. Police presence would be immediate due to constant patrols over the bridge. 4. the construction company would press charges were anyone caught. They're kinda funny that way. 5. I would expect that any equipment (sound, generator, turntables, etc. ) would be confiscated.

Sure, the above is a worst case senario but given the proximity to downtown and to cop shops, I feel it is pretty realistic. Conceptually, this is a great idea that would be a fantastic experience were it not to get busted. However, I think for it to be successful, it would need to be a permitted event rather than commando style. One possiblity might be to contact the construction company and see if they would be willing to allow the event to happen on their site. Highly unlikely due to insurance concerns but stranger things have happened.

Obviously, this is just my opinion...

ken