This doesn't work: hatches activate instantly. The dwarf will step onto the plate, and the hatch opens immediately. Without a path, the dwarf will cancel the job and wander off. Replace the two hatches with a drawbridge that raises towards the plate. This will cause the dwarf to step on the plate, and walk forward. 100 ticks later, the bridge will raise, forming a wall, and negating any "fling" from the bridge, due to the 1x1 space. With an empty space there now, the dwarf falls straight down, and the bridge resets.
Good luck doing this in time to get all your dwarves inside during a siege.
The point's not to drop them-- it's to summon them. (You're right, though, that a bridge based construction similar to this could be used for dropping dwarves, if that's your thing. I'm pretty sure you'd need to reload it every once in a while though. Still, it would be more useful for most people than my device.)
What you do is build several of these deep in your fortress. Depending on population, you might need a lot, but they're simple so you can do that. The dwarf gets there, cancels the job; 100 ticks later, the path opens up again, and some dwarf-- maybe the same dwarf-- takes the job. You need a number of these proportional to your population, but every idler will get grabbed. The further you place these from where you don't want your dwarves, the better.
The dwarves might wander off when they lose path, but the job is high enough priority that as long as one of these is unclaimed, they won't wander far, and you've already done your job by bringing them to a certain location.
It'll work nearly instantly-- almost as fast as you can pull a lever (and consider a high-speed creature-based switch if that's not happening fast enough for you). Remove construction is a super high priority job.
I envision using this to control year-long flow through a fortress. With slightly more than your population, you can bring your entire population to any particular point in your fortress just by flicking a switch. As soon as you want them to return to work, you close the hatches.
EDIT: If you're not so hot on the idea of idler/child summoning, maybe you'll see the value in a small variation-- I call it the Undump. They're gonna be big one of these days. One day, you'll say things like, "Yeah, I used to play DF, before undumps came along and ruined stockpiling."
#####
^shs#
#####
h is hatch; ^ is pressure plate, citizens trigger, linked to hatch; # is wall; s is stockpile (same type)
Single tile stockpile while preserving dump functionality, without having to mark anything as dump. Tested. Works. No bins necessary, etc. Works great for mechanisms, doors, bags, corpses, hair, skulls, ammo, even stone should you feel the urge for a clean fortress.