Ok, I'll try to explain the intent better.

Example:
A mod wants to add a villager, a Brewer. He sells potions. When a new villager is born, he should not be allowed to become a Brewer unless there is atleast one cauldron in the village. This should be evaluated every time a new villager is born, as the player might have added or removed a cauldron from the village.
This is just one example, it's up to every mod to decide what (if any) conditions they want to put on the spawning of their villagers.

If the idea behind this is something you guys like, I'd be happy to continue refining the code and removing the things that "looks like ass". Or any one more skille that me can take the idea and make a proper implementation. If how ever you consider this a terrible idea to begin with, I will just retreat into the shadows and never speak of this again.