I like the idea of an outside vent, and unfortunately, I think an insert of some kind. I can't help but first think of the goofy look of a wood fired stove sitting half in and half out of the fireplace. Surrounded by a few tons of river rock. That said, after a couple days of a couple degrees, it's either that or the tent in the basement.

*outside intake. The vent would just be the chimney.