My container of hard, dry brown sugar was dumped into a large Ziploc bag with a tiny bit of water and placed in the microwave, after about 20 seconds or so it was softening up. I kept massaging the bag and re-microwaving it until it was pretty well malleable. Then I put the whole bag, tightly sealed (with the soaked clay bear) back into the plastic container...I'm thinking that'll keep it from drying out.
Kim, I love your idea and I'd use it if I ever had marshmallows in the house.