Here's a destructible skybridge I created on a map called Velodrome. The skybridge is the location of the rocket launcher and can be seen from most of the rest of the map. Activating its collapse can't be done from within the skybridge, but can be done from most places in the open areas of the map. I've made it so you can't really escape falling if someone catches you in there unless you have a jet pack. The rocket launcher spawn is set to two minutes (a little quicker than usual) because I imagine successfully aquiring it will be a little harder than usual. Will need to do some playtesting to confirm that. I may also restrict views to the activator if it becomes too difficult for anyone to get the rocket launcher. Here's a view from inside the skybridge Skybridge collapsing Click here to see the collapsing skybridge in action. Click here to download the Velodrome (test) map.
Looks like you could still crouch walk on that ledge to the right in the first picture even after the bridge is collapsed.
How does this collaspe? It looks like this this could be acomplished by land mines at the bottom of each crate.
The crates are currently sitting on fixed fusion coils. I haven't messed around with land mines yet, but I imagine they would work as well.