I'm sure multiple people have thought of this, but this is my rough plan for burrows and the fortress entrance.
Firstly, all dwarves will be split into three groups, Military, Civilian Assistance and Citizens. Military will be the military (duh). Civilian assistance will be those dwarves who need to access the entrance are - engineers to clean the traps and worthless soap-makers to haul the goblin-socks. And Citizens will be all the lucky folk who have no business ever stopping in the blood, vomit and gore soaked entranceway.
In times of trouble, the Civilian Assistance dwarves can be removed from the entrance area access list - I'm hoping there's an easy way to do that without redefining burrow boundaries. That way, nobody will rush out to clean traps or grab socks while the seige is still in progress, and when the siege is over you can let expendables out to clean things up without disrupting the workflow of your more valauble dwarves in the sock-rush or exposing valuable dwarves to the possibility of another ambush.
The other citizens can all path *through* the burrow to get outside assuming they have need for that (Probably make another Outside burrow defined for the brave topside-going folk). But they won't take jobs in there.