Code Rule 9B-3.0472, Carbon Monoxide Protection,
(3) In new construction, alarms shall receive their primary power from the building wiring when such wiring is served from the local power utility. Such alarms shall have battery back up.

Do we extrapolate from (3) that in other than new construction battery only devices are permitted.

As and example a fire place is added to an existing dwelling. Can the fire place installer just install battery only detectors and forego the cost of hiring an electrical contractor to hardwire new CO detectors.

That does seem to be what it says.

Does it say anywhere other than new construction that a CO detector is even required?

I suppose this comes up in plan review for garage, fireplace and gas permits. It will just be another checkoff on the permit application.
I am still stuck neck deep in the permit process (as a "customer") with no end in sight. I guarantee this will be dealt with in the lobby of the building department.