During all of this current_soul searching, has anyone happened upon the values required to make a creature into a functioning member of the fort? When I use warmist's spawn-unit script to make an animal, it's fine for hostile ones but unreliable for friendly ones. Giving it the player's civ_id will make it appear "Tame" on the units list (and keep it from attacking) but "Not Tame" on the z/Animals list (which prevents it from being a pet) even if I gm-editor its training level. I recall reading that the creature can revert to hostile when the game is saved and re-loaded, but I haven't witnessed this personally.
Comparing the gm-editor screens for a natural specimen and a spawned specimen hasn't gotten me anywhere, but that's because I'm not familiar with the object model. If someone can point me to the proper attributes and values, I think I can make the Lua code changes necessary.
The immediate use of proper fort membership would be to enable war-training and such for spawned animals, and I have plans for assigning specific creatures as pets of specific dwarves in a later expansion.
And, it doesn't get said nearly often enough that DFHack and the other third-party tools are awesome and invaluable additions to the DF game and community.