I have hatched runner ducks under my broody chicken and she is the best duck mum ever! chicken eggs only take 21 days to hatch and mallard-derived ducks (except calls I believe) take average of 28 days. My chicken coped fine with the extra burden. Since muscovies take 35 days, it might be a huge extra effort for the chicken, but so long as you ensure she gets enough food and water, she may just cope fine. I made sure my hen got off the nest twice a day for exercise and refreshment so her condition did not deteriorate too much, and gave her lots of high calorie treats like dried mealworms. she did lose a bit of weight though and I am still feeding her up now and they are a few weeks old. she also went straight into a moult afterwards. I say give it a try with a trusted broody and just give her extra TLC and treats all the way through. If she is a good mum she will keep sitting until they hatch.
Perhaps someone else will chip in some advice who has successfully hatched muscovies under a chicken. I hope so.
