My guards need something better to do than make my nobles happy with hugs and justice.
Sigged.
On topic: I concur that this is a military function and would be well assigned to military dwarves. It could be within the purview of the Captain of the Guard... but having the fortress guard do it would be an excellent idea too.
But then, the FG will be a squad under the CotG in the upcoming version. So maybe just having any squad assigned to the area of a checkpoint, handle the lever as appropriate, would work.
I absolutely *love* the notion that wagons won't instantly bypass a site because the depot is momentarily inaccessible. They might have a fairly short tolerance before they turn around and leave, if they can't find a path at all, but they should at least check in. Even if I then open the gate myself.
Also, having wagons that don't have a clear path instead follow a road until it comes to a halt(or rather, a few squares short of it, to be sure the guards don't wander under a bridge) would be a good way to handle that part. Having wagons follow a road in general would be very welcome, and would make it feel a little less tedious to build them, knowing that they do more than just keep trees from growing.