As I recall, when a gobbo gets caught in a cage trap, he automatically drops his weapons, including crossbows. Most of his clothes too, for some reason.
That hasn't been my experience. What I do is:
catch goblin/other foe (I usually play genesis) in cage.
once the cages are in my animal stockpile, I build (b -> j -> x, select "goblin cage") the cages all in the same room, surrounding a central 1-tile garbage dump.
Since part of my process is to reclaim the cage after the assault is over, most times I don't need to reclaim the equipment, but I generally do anyway, just to be safe (d -> b)
after reclaiming everything, I then go over them with the dump command (assuming you didn't press escape, just press d)
Once the dwarves are done swarming the cages and
stealing confiscating all the goodies, I remove the dump order on the cages (d -> b -> D), and deconstruct them. They are now ready for execution.
To deal with tricksy thieves, I built this:
Upper level:
XX+XX
X.+.X
X.=.X
X...X
XXXXX
Middle Level:
XX+XX
X.+.X
X.O.X
X...X
XXXXX
Bottom Level:
XXXXX
XXXXX
XX.XX
XXXXX
XXXXX
Note: Central hole may be as deep as you like. I find 10 z-levels never fails to a-splode the gobbos.
Key:
X = Wall
. = Open Space
+ = Floor
O = Pillar
= = Floor created by pillar
To use:
1. (re)Build pillar.
2. Build goblin cage on top of pillar.
3. Link lever (not shown) to cage.
4. Remove pillar (d -> n) IMPORTANT: Wait until the mechanic is done linking the mechanisms to the cage, at least. I am sorry, Urist.
5. Pull the Lever.
6. Watch the gobbo disappear down the hole. I've had parts splash up 4 z-levels, after a 10 level fall.
7. Are there goblins still in cages? Then go to 1. If not, deconstruct the lever to recover all the mechanisms.
Can't remember who suggested this method to me, but I found it here on the Forum. Thanks!