You could try "303", an aerospace formulated protectant. As John Proch wrote, once rubber (and other synthetic petrol based not-really-rubber-as-in-from-plants rubbery stuff) goes, it's gone. I've used 303 in the Navy and on my own gear, from Southern California to jungle tropics to North Atlantic winters and it does a great job preserving (extending the life) all sorts of coverings and seals. It's SPF 40, and reduces the UV damage that is the main cause of much of the deterioration.
see: http://www.303products.com/
Glycerine, btw, is a common ingredient in soaps as a moisturizer. It tends to vegetable based and is a natural moisture protector.
Bob Hillery
Stratham, New Hampshire, US