Hi askomiele ... it's no problem with zippers and buttons!!
The best way to do it is to turn the jeans inside out and went placing jeans through the wringer, fold them in half at the waist with the button and zipper facing inside. This way, the cloth of the item you are wringing "protects" the zipper or button and the upper roll on a Maytag is a "flexible" roll. It "gives" with the buttons, zippers, hems ... any thick item ... and provides further protection.
When wringing shirts, you do the same thing. Fold the shirt in half at the collar and run it through the wringer with the buttons protected by the cloth of the shirt.
Geoff