Friend (a machinist) tried that years ago. Then he checked out the
little woodstove I had, made a copy of it, and forgot all about the
fireplace.

Because it was (and is) obvious that a fireplace is a ventilator, not a
heating device. Nice to look at, but so is a woodstove with lots of
glass in the door(s). All that airflow up chimney from fireplace is the
efficiency-killer, more so the greater the difference between indoor
and outdoor temp, not to mention the hours the damper is open while the
fire dies.

Thin-wall tubing sitting right next to fire will not last long, either.

Sorry 'bout that.