There are duckxgoose hybrids, i.e :
Poulsen, H. 1950. Morphological and ethological notes on a hybrid between a domestic duck and a domestic goose. Behaviour 3:99-104.
Checking Gray's Hybrid book shows alot of hybrids between ducks and geese. Muscovies are listed as having hybridized with 16 other species, including European shelduck, Egyptian goose, swan goose, spru-winged goose, and comb ducks. Delacour is the source for Resolution's reference to domestic muscovies including comb duck genes. I can find no other reference, and checked the comb-duck duckling description to see if that could account for the dusky ducklings, but it doesn't.
I'm not sure of the fertility of all the hybrids, I'll have to check the original references, and that's often lacking because most don't try to breed the hybrids.
Clint