I embarked with some complete sets of copper armor (leggings, breastplate, helmet, boots, etc.) and dropped my militia commander out of a hatch onto a 10-spear trap while wearing a full set of copper armor. He came out legendary fighter, hammerdwarf, and shield user. He was not injured, only stunned, possibly because it was a 2-Z drop onto a soft soil floor. The copper armor blocked all of the spear attacks (one of the spear attacks was blocked by clothing under the armor).
Another thing I theorize may help avoid unintentional injury is I used a hatch instead of a retracting bridge. Retracting bridges fling whatever is on them, which might make the fall worse. A hatch is harder to get the Dwarf to stand on, but doesn't fling him. To get him to stand on the hatch, I built a medium length 1 tile wide corridor with a hatch in the middle and doors immediately adjacent on either side of the hatch. (When I tried this without the doors to lock him in, he simply moved over when the hatch opened instead of falling.)
I wanted the militia commander to be in a squad / on duty to ensure he'd be properly armored. However you can't station a squad on an exact square, he'll only go somewhere close by. So I alternately assigned him to move to a station on one side or the other of the corridor until I managed to catch him in transition between the two doors, which I locked just as he was passing the hatch. Then I had someone pull a lever to open the hatch. I think this could be automated by making a squad patrol through a corridor on a loop and have hatch(es) open and close randomly on a repeater.