I just found out I can revive thawed and dried out onigiri by putting a little water and salt in a mason jar holding one rice ball, shaking the whole thing to get the rice moist all over, and microwaving the open (!) jar and its contents for 30-45 seconds. Voilà, warm and sticky-moist onigiri. Of course not as good as fresh ones, but still a good way to rescue ones that dried out while in the freezer. Not recommended if you can’t face the risk of your pretty rice ball falling apart more easily.