I rather like the goblin based party ending, but actually releasing one is too messy. I'd recommend placing the goblin in an enclosure of bars or windows, and then surround that with lever-linked floodgates. Pull the lever, the floodgates drop, everyone goes "Aaaaah goblin" and stops partying.
...windows might be the better choice, in case there are marksdwarves attending the party. Replacing the goblin each time would be rather tiresome.
Well, dwarves can see (and be scared of) creatures that are on a different z-level than themselves, so how about something like this? Entirely theoretical, of course, since I haven't used statue gardens and stuff since the 2D days.
z+1
....................
.......bbbbb........
.......bwwwb........
.......bwgwb........
.......bwwwb........
.......bbbbb........
....................
z0
....................
....................
.........s..........
.........S..........
....................
....................
....................
. = floor
b = bridge (diagram sucks, but basically four 4x1 or 4x2 bridges in a square around a 3x3 area of floor tiles)
w = window
g = goblin
s = statue
S = support (otherwise the goblin and windows would crush the dwarves below)
Basically, with this setup, the idea is that when you want a party to end, you pull a lever which retracts/raises the four bridges (for obvious reasons this needs to be located somewhere else), thus revealing the goblin above. Surrounded by windows it's totally harmless, but dwarves get scared, cancel their party, and return to work, at which point you can pull the lever again and the goblin's hidden from view once again. This doesn't leave you with any ugly bits along the side of your party area and it doesn't have the goblin being dropped from one level to another (and having to be lugged back up to the top level to reset the whole thing), just a support for the nine floor tiles on the level above that are unsupported when the bridges are retracted/raised.
Like I said, it's theoretical. Dwarves do get scared of creatures on z-levels above/below them, even if there's no way of the creature being able to touch them, but whether something like this works adequately enough (there's obviously a blind spot of at least a 3x3 area and possibly a 5x5 area directly below the goblin where the dwarves would be unable to see the goblin above and I don't know if they'd be scared until they moved out of that little area) remains to be seen.