The magna carta is indeed a deadly piece of text.
Personally I favour imprisonment, or creative ways of goblin destruction.
Yes, but if you're using the magna carta, you can't imprison someone without having a jury of their peers find them guilty. Have you ever tried to get 12 goblins to show up for jury duty? Which is why my forts toss the magna in the magma and just get on with dealing with prisoners in a practical manner.
OP: Use 'i' to designate a 1-tile zone near your goblin cage(s) and set it for garbage. Use d-b-c and select the cage(s) with the prisoner(s) to unforbid their possessions. Then use d-b-d and select the cages to mark them (and all of their contents) for dumping. Now use 'k' to go to each cage and hit d to unmark it for dumping. You've now marked all of the goblins' equipment for dumping. Your dwarves will come and strip all of the goblins of their stuff and put it in the garbge zone. Use d-b-c to unforbid it so your dwarves can do somethign useful with it.
Now you have two choices for releasing the goblins.
1) Build the cages (use b-j) in a room with a door (your live training room) and link them to a lever. Put you military melee squad(s) in the room, lock the door, and pull the lever. The unarmed goblins will be released and your troops will quickly dispatch them, getting experience in the process. Have a hospital set up, just in case of training accidents.
2) Build a shaft over your training room and designate (i) a zone at the top of the shaft and make it a pit (p). Then use 'P' to select the critters to toss in the pit. Select your goblins. Your dwarves will come to the cage, grab the goblins by their ears like your schoolteachers used to do, and lead them (ow! ow! ow!) to the pit and drop them down. If the pit is less than 26 z-levels deep, your military should be waiting at the bottom. Actually, have the military waiting off to the side, so the goblins don't land on them. If it's 26 z-levels or deeper, you just need plenty of haulers, since the goblins will explode on impact.
There are variants to the process, including just removing their weapons (you have to use the z-stocks menu to do that) so they survive longer against your troops so as to give more experience. But the above will get the job done.