Use command to treat a player created structure as natural

In Minecraft, the real objective is to create whatever the player can imagine. Sometimes we want to create our own versions of naturally generated structures, such as fancy witch houses to elaborate forest mansions.

The problem is that mobs that only generate within a specific naturally generated structure will not do so in a structure created by a player. With loot tables and commands we can recreate naturally generated chests, but if you want a specific mob to be present you will either need to use a monster egg (in which case it might despawn) or you will need a monster spawner (in which case too many will spawn). 

If, however, we could use a command to set the bounds of a user created structure to be treated as if it were a naturally generated structure like so:

/setStructureID forest_mansion X Z X Z X Z X Z

You would just need to know the 4 corners of the structure, but it could be a square or a rectangle and then the game would henceforth treat that area as if it were a naturally generated structure of the assigned type.

Mobs that don't respawn - like Evokers - should at least spawn once with the structure defined, as if it had just generated. For mobs that respawn like Witches this feature would be all the more important.