Using WorldEdit I can create a mobspawner of arbitrary kind (except that it still spawns client-only ghost-pigs that haunt me 'till I reconnect). Unfortunately they work too good so rooms get overcrowded. So is there a bukkit plugin to set spawners up such that they only spawn a limited amount of mobs into one room?
Creaturebox has exactly what you are looking for. It allows you to set the mob in the spawner, adjust the frequency of spawns, change the maximum spawns from that spawner, and even turn it on and off using redstone signals.
There are also NoSpawn and EntitySuppressor, both of which have to be updated to 1.3 though. I'm also looking into Mobs which seems to offer some conditional spawning, but I don't have enough time to test that at the moment...
Since I have currently only one spawner of each kind, HeavenlyMob which provides a per-world limit, is a partial solution. But still I'd appreciate better answers.