I played with this idea once, and as i happen to smoke, i've got plenty of ash on hand. Was using it at the time instead of flock for some bases. Works pretty well, but you need a few layers if you're looking for depth, as it can end up pretty fine after getting mushed up a bit.
Although i *seriously* don't recommend smoking just for ash. Since you've got a fireplace anyway, that'll make more ash in a day than I could in a month