Oooh, interesting. Not tried it - my ducks have shown no signs of broodiness - but the main problem I can think of is that they might not be gentle enough with chicken eggs. They're much easier to break and ducks seem to be a bit clumsier. Maybe give it a go and see - would love to hear how it goes! Oh, and not sure how confused a chicken would be at being raised by a duck - would it try and make them swim? That might be an issue too.
H