I made an easter cake this year with sweet potatoes.
i was actually really lazy and used a yellow cake mix then I added:
1 cup of sweet potato puree
1 tblsp oil
1/4 cup water
6 ounces of peach yogurt
i always add 1 tblsp vinegar for moist-ness
pour into greased pan/s bake at 350 about 20 minutes.
i think you could substitute pumpkin or another "sweet" veg puree. I believe you can add pureed beets to a chocolate cake and it hides the taste. I've also had success hiding spinach puree in bluberry preserves when making oatmeal bars.