To the original question of this thread, I think I might have discovered something semi-useful. I need to experiment with it a bit more though and it's more useful in relation to military, but it is keeping them from starting the bum rush too early.
I have an outer courtyard that can be closed off at both ends. It's 1 Z-level below the ground level and my door. (In the pic it's being "washed" with magma following a siege). Currently I'm letting in a decent number of gobbos and then closing the outer door and letting my melee dwarves out to slaughter them. This keeps my dwarves from having to deal with the 200+ siegers I'm getting all at a one time, and getting tired/killed. The inner gate of the courtyard is on ground level, but then my main hall (pillared open space) where I station my melee dwarves to wait is -1 Z-level again. Same as the outer courtyard. So effectively I've got a dike between my siegers and waiting dwarves. Since the dwarves don't see the the gobbos until they get to the inner gate they seem to hold position nicely until the gobbos are right on top of them. Then they rush up on the dike/gate together and out into the courtyard for the slaughter, while I simultaneously close the outer gate.
Anyways, it needs some more testing, but it seems that dikes, that block vision, are potentially useful in helping your dwarves stay put until the enemy is close. Potentially it has civilian applications as well, but would require a lot of attention to overall fortress design.