The Implementationof an air ventilation system could be highly abstracted, such as not needing any flow calculations etc.Smoke going up in chimneys would be way cool, and easy to implement and calculate for the computer.
I like this part of the suggestion, at least for workshops. The calculation could be made pretty simple:
A tile either has smoke or does not. Fueled workshops produce 1 unit of smoke for every item created. The next unit of smoke is always generated in the nearest tile with no smoke in it. If the top of the workshop is connected to the outside air, the smoke is not generated in the fort, but instead generated in either the shaft or the air above the shop (whatever is closest). Smoke is not generated if it reaches 3 tiles of open (above surface with no walls touching it) air. It would probably be wise for smoke to "expire" every so many turns.
So, if you build a workshop with a smokestack and it is in continuous use, the stack will always be full of smoke and have a 2x2 plume coming out the top. If you don't, your fort will slowly fill with smoke and give bad thoughts as well as possibly suffocate.
This would also allow early forts to get away with not having ventilated workshops, because as long as they aren't being used, they won't generate any smoke. Once production goes full-bore, shafts (or being built on the surface) will become a requirement.