I have no idea, but possibly you could contact a textile museum to see if they could weigh in on this question. It would surely prevent the growth of mildew on your quilts, but maybe it would do something to them that wouldn't be good. I think that shrink-wrapping them with a seal-a-meal type device beforehand might be a good idea to really protect them from any transfer of moisture or odors - but I don't know of a device like that big enough to accommodate, other than the bags you shrink with a vacuum cleaner, and I've never found one of those that's reliable.
I definitely agree with others that jugs of water would be a better idea for filling up the space. The best thing about them would be that they would slow down the thawing in the event of a power outage. I don't have much room in my freezer, but try to keep some smaller containers of ice in there. They're great to put into a cooler for a road trip or even just to take cold things home from the grocery store in summer. Other things that I use to fill up the space are staples such as flour, cornmeal, nuts and pasta. Well sealed in plastic, they keep much longer when frozen. When I cook pasta I save energy by cooking the whole box, even though there are only two of us. The part that doesn't go into my recipe immediately gets a little dash of olive oil stirred in to keep it from sticking together, and then it goes into a freezer container. It saves time later.