FINALLY!!! Posting to bump this. Caught a pair of ogres. One had killed an FB so my marksdwarves won't shoot him but the other one they are going to town on. And it works! Mostly!
Issues:
The first dwarf to start firing got in 2 bolts without missing, missed the rest of his quiver. Roughly the same pattern for all dwarves firing from the top.
On the collection level, one dwarf has loosed a quiver so far, out of dozens of collections by marksdwarves. He is the most elite of my marksdwarves, and has also got the highest observer and discipline skill in the fort. So - highest observer, archery, marksdwarf, and discipline skills (legendary+3 minimum in all) means they will fire from the collection level. I think. Haven't had time to prove it beyond all doubt.
Marksdwarves collecting bolts get a job cancellation error (doesn't matter, they still pick up a full quiver), then run up to the top to fire. Not practically harmful, just means you get an annoying error for every 25 bolts fired.
Other than that, perfection. Really not that big a deal to set up, and once I get undead invaders my bait shouldn't get derpshotted every now and then. That it saves the vast majority of my bolts while letting my archers loose on live targets is the important thing, which I can't say for all my other live fire schemes. All I need is a way to make the lower levels inaccessible to marksdwarves, I think - to avoid the bait getting a faceful of bolts every once in a while and to avoid the cancellation spam. A possible solution is simply having a few dedicated haulers and a massive stockpile on the top level.
Basically: thanks Qrox, this is a miracle of archery training. I love you. PUT THIS ON THE WIKI.
EDIT: sorry about the humungous images, it's late and I have no time to crop them.