Zucchini is 95% water, so the main issue for bread is that you're
adding something close to pure water. By starting with a zucchini
puree, at least you could try to lower the water content somewhat. The
other question is whether the modest zucchini flavor components can
stand up to the flour and yeast. The technique of adding eggs, sugar,
and oil is probably to compensate for the small contribution of the 5%
of the zucchini that is not just plain water.

Ron