When I used Heng's on our Southwind, I sealed the edge of the roof with Eternabond. I put part of the E/bond on the rain gutter for a better seal.
I don't know why your idea wouldn't work. Just make sure the metal is clean before applying the Heng's.
FWIW, I used 2 1/2 gallons of Heng's to apply two coats on 30' of roof. It's nice stuff.