Scott:
IMHO, 426 does not apply. As to 427, yes basically as you described.

GFPE for the slab heat would be a design issue, dependent on the plans, and the attached specs to said plans, as to IF GFPE is required by the design team.

As to 'stand alone' GFPE/GFI devices and equipment....yes they exist, and are available. I recently posted a link on this at Mikes site regarding pond aerator (480v) equip. At this time, I do not have that link near me.

Stay safe..in the office or out!


John