Thanks all for the input and suggestions. Many good thoughts there.
I'm thinking along the lines of having a totally separate firebox for the propane section with it's own fresh air intake and exhaust. Yes, the stove would be freestanding near the center of the house. I like the idea of incorporating the burner at floor leve, below the wood firebox.
I really haven't gotten as far as discussing it with any masons, and had just had the initial brainwave.
I was sort of half expecting that someone somewhere has done it already. (Nothing new under the sun and all that.)
I'm on the same page with some of you on the liability issue. This would have to be a burner which was in some way "legit" for the application seeing as it's for a customer and not for myself. Also, installed and designed with the blessing of the mason executing the work. Nothing janky for customers! Myself, that's another matter...
Caveats aside, the difficulties really don't seem to be insurmountable and seems to have the possibility to be a worthwhile idea to pursue further.
Maybe this will go somewhere in the short term, or it may end up shelved in the mental warehouse for later...