We like the whole cranberry sauce right out of the can on sweet potatoes. I'm not sure how much sugar is added to those cans - I wasn't McDougalling last year when cranberry sauce season began.
Salsa, especially salsa with lots of cilantro, works great.
My husband likes stewed tomatoes on sweet potatoes - not a fan myself, but it is healthy. He also sometimes uses ff barbecue sauce, which is surprisingly tasty. Oh, and he uses tamari sauce.
Any of those Asian sweet-hot-garlic red sauces are really good (my favorite), and most don't have oil.
Mr. Spice makes several McDougall-legal sauces, including a curry sauce I've just ordered and plan to try out on sweet potatoes:
http://www.mrspice.com/posics.html