Kobolds don't trip pressure plates or traps.
Wasn't sure at the time of writing, but, given one had apparently been cage-trapped, I thought I'd run with the idea. Cage traps: yes; Weapon traps: no?
Still, the next goblin you cage would be a better ironic canditate for powering the "on/off" spikes or other reciprocating trap.
Something like the following, maybe:
abcdefghijkl
'''''''''''''''
'''''''''''''''
1''''#F#######''
2''''# F^ F''
3''''# #####F#''
4''###F# #^#''
5''FCF # # #''
6''###^# # #''
7''''#F##### #''
8''''F ^F #''
9''''#######F#''
'''''''''''''''
'''''''''''''''
Assuming I've not made any errors (in understanding the mechanics/game behaviour or in the presentation of the diagram or following description), the "C" at b5 is the hostile's original cage, with floodgate (or other suitable switchable barriers) controlling access to the immediate outside world (a5) and "inner circle" (c5), controlled at leisure for the purposes of loading and releasing. The cage can be conntected to the same control as the c5 gate, probably. Start with all 'outer' gates closed except k2 and all inner ones closed except e2 and the one at d4, then let them loose. Hostile heads towards 'escape' outside k2, wanders over e2 pad which closes e2 and k2, but opens j3 and j9 so it'll keep on going round to the new exit. Hostile then activates j4 which closes j3 and j9 but opens i8 and c8, to similar effect. Stepping on j8 closes i8/c8 and opens d1/d7. d6 sets up the cycle again. d4 closed, with a5 and c5 opened and b4 reset as a cage trap will bring the event to an end. But while it's working, if you've set the pressure plates to perform other tasks (or linked up other ones, not shown), FNU!
A variation on that can be made which doesn't need so much of a micromanaged reset, and doesn't snare the hostile in the vicinity of the b5 zone, just confine them. On/off control of this process can easily be made and configured with pressure plates and a single external "activate/deactivate" switch (or pressure plate from another goblin-powered ring to form an appropriate cascade of logic!) If you're not as bothered about this automation, the c5 barrier can be ommited. As long as a5 remains closed (i.e. you're not wanting to reclaim the original release cage, or set up anything else in there) there's no reason for it.
Obviously the reaction times of the floodgates is a factor (as might be the possibility of crushing the 'worker'?), so either longer legs of the 'circle' (with the added advantage of giving a longer period cycle, if required) or using hatch-covered pits (and an appropriate set of steps back up for those caught on them, just in case) would be a better barrier. If you want to bash the invader around, 'launch' them around parts of the circle with lengthy bridges with an appropriate direction of raising, and see how long they survive...
Of course, my original thoughts were just about a reciprocating method ("Run run run, towards the western exit! No, that's closed now. Run run run, towards the eastern exit! No, that's closed now. [...repeat...]") which would be far simpler, but not as much fun to look at. And with a circuit, one can coral a
number of hostiles through its system (inserting new ones all the time), where the slower ones get caught out by the barriers closing behind the faster ones. (Though I'm sure you'll have realised that this causes problems with multiple plate activations, without a suitable 'buffer' mechanism to stop that. And also some might possibly become caught
behind closing barriers, and potentially escaping from the anti-clockwise exit when the leading ones reopen them, near the end of the next full cycle.
Less symmetrical (or just more convoluted) systems with other than four 'modes' can be made... Never mind branched ones (figure-of-8 two circles joined being the simplest) with carefully equal-length exits (or externally and/or chaotically influenced escape-paths, to draw the hostile round one circuit or another) which is of course the start of any large-scale flow-based computational device.
I really ought to try this, you know (I have an excess of caged hostiles, a whole stockpile full of them, including a couple of elven members of the goblin masses). But I'm not going to be able to play my DF at home until next week. Darn, why do I always get ideas like this when I can't do anything about them?