I am pondering adding a wood burning fireplace in my living room, which
is a cathedral ceiling, with a 2x12 construction floor.

First question: ever seen custom zero clearance fireplaces? It would
eliminate the need to bash a huge hole in the floor and pour a footing.

Second question, if I am forced by esthetics to go totally masonry, has
anyone experimented with thermal breaks, above or below? I find little
discussion about this on the web, and it would seem a major source of
heatloss. I imagine the chimney could be steel even on a masonry
fireplace, making it easier. The base could be insulated to some extent,
but I was thinking with the fairly short height to the floor[~3 feet
from the ground to the floor surface] one could pour a footing and
support the fireplace structure on heavy steel columns, cutting the
thermal exposure to a fraction.