I've got tons of caged trapavoid. Don't want to test, but I'll email a save to somebody if they just need caged trapavoid to test.
One of the problems that might come up with this is that it's really, really dangerous to cage dwarves. Right now, caged dwarves don't show up on any assignment or pitting screens, meaning the only way to get them out of the cage that I can see is to build their cage and link it to a lever. In the time it takes for that to happen, it's really easy for them to get dehydrated and die.
Now, the only way I can imagine this being useful is for fighting a bunch of trapavoids. FBs aren't so tough except for their syndromes, and caging the wounded isn't going to protect against that. There's of course another large group of trapavoids that you might think about building an arena for, but those guys are so huge that they don't tend to cause a lot of passed out dwarves-- dwarves just kind of die. And cages still get covered in dust, and melted in death explosions, and catch on fire, and fighting those guys takes a long time, and it's not so simple to just get them to go someplace else so you can recover your caged dwarves.
Wait a minute.... You know where I might do this? On a marksdwarf wall. It would be effective enough there. Any to pass out get caged.