I can happily confirm that replacing milk with zucchini minced and pureed with a bit of water WORKS. I did it for my chocolate cake. It tastes and feels the same as it did before which is great because milk is can get costly and zucchini is better calories wise. Next year, I am so planting zucchinis for baking.
Of course, if I am making any other cake, I have to peel the zucchini or else it will be green.