Yes, leave uncovered so air can circulate (carrying moisture away from machines). Any covering will trap moisture next to the machine.
You can purchase silica gel at JoAnn Fabrics and create little packets of moisture trappers (similar to those found in shoes, etc.) with inexpensive coffee filters. Youtube has some videos on how to do this. Gun owners make up these packets to keep stored ammunition dry for long periods of time.
Edit: On second thought, the packets won't do any good in an entire room like that. Whatever is being kept dry (such as ammunition) needs to be in a container. The silical gel keeps the interior of the container dry. It wouldn't be enough for a whole room.