Since Mastercoat Permanent Rust Sealer and Mastercoat Chassis Black Finish Coating are "moisture cured"...that means the more moisture the better, right?

NO. Mid-range relative humidity (50%) is perfect for these coatings. You don't want them to dry too quickly, which is what happens when you use them in high humidity. When the humidity is too high, you risk brush marks, bubbling, and poor adhesion.