So for the movie we'll be filming at least one bridge guerrilla suspension (shhhh!). The trick of course is doing it quickly without getting yourself arrested, which means that you (a) have to be able to set up very fast, (b) get the person off the rig fast when you decide to, and (c) break down the rig quickly.
Unless I'm missing something, the easiest way is probably a vehicle based system. I've whipped up this idea for a bar that attaches to the top of my Jeep's roll-cage. It's super-simple: just a triangulated bar with “hooks” on one end that grab around the edge of the roll-cage and are then held in place with pins. Optionally the whole thing could be attached to the winch as well.
So when you're driving it mounts lengthwise and stays in “road legal” territory. Then you pull up onto the side of a bridge, spin the beam ninety degrees, lock it into place, and you're good to go… then yank the person back up (easy since your mounting point is already eight or nine feet off the ground; just pull the pulley to the top. Pop out the pins, spin the beam back, re-lock it into the driving position and disappear into the night.