@grouter:
Blocking the airflow should not cause too much problem if you account for it. Same issue exists with zone-control heating. You need an extra air vent somewhere (zone control usually puts one right by the furnace) that vents it to maintain a "good" pressure in the ducts. So if you put smart vents everywhere you could have another in the basement, near the furnace, or some central room that could just be set to release far more air than the rest.
Although this also reminded me I have more vents in the basement that would have cost more money...