I have heard of it being done before. In fact the woman I sold my pair of calls to told me a story about her cochin hen who hatched and raised call ducklings. Everything went great until the day the ducklings dove in the pool, the hen darn near lost it and dove in after them almost drowning herself. Luckily the lady was right there to pluck poor wet mama hen from the pool.
mine hatched two last summer, a crested cayuga and a campbell. but, this was in july in missouri with 80-90% humidity outside. since ducks need ALOT more humidity than chickens you may have to mist them toward the end. I would post on the duck board and ask them