update MobSpawnControl seems to achieve this. (Note to self: CustomSpawners looks like fun).
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.