Hmm--I don't really know if the ducks would learn to stay away or not. Our neighbors have dogs in a fenced back yard, and the ducks have never tried to go in with them. My ducks totally free range during the day, but they have a predator-proof enclosure at night. They range down on an adjacent lake, into the neighbor's yards, across the street, up and down the drainage ditches, etc. On the rare occasions that there are large dogs loose in our neighborhood, I've lost ducks to them, but it's always been in our yard, so I'm sure the ducks are not chasing the dogs!
Still, I just really don't know. If you've had bad experiences with the muscovies before, I would be hesitant to try again, but maybe you'll have better luck this time. It's a learning process, anyway--I had a lot of losses early on until I figured out how best to protect my birds.
Good luck, whatever you decide!