Wait until she gets off the nest to eat! Add a few fake eggs if you want. Tractor Supply sells the fake ones.
Off on a totally different tangent....can you break a broody duck like you do chickens? Oh, and your advice about keeping a broody hen in a elevated pen works like a charm. It was less intrusive as I did not have to play cop and keep entering into the coop to shoo her off the nest box.