Under Article 424.38 Part B it states the uses which are not permitted. It states it cannot be installed over the top of walls where the wall intersects the ceiling and cannot be installed under or through walls.
Nowhere does it state that the heaters cannot be installed in walls.
If the electric underfloor heating mat were installed on gypsum board or other fire retardant material with self-leveling compound, adhesive and tile over the heater, would this be allowable under NFPA70?
The heating cable itself being completely earthed including coldtail, protected by a GFCI thermostat.