I checked the Edmund Industrial Optics catalog, in the mirror section. The aluminum coatings are the most efficient at UV, down to about 350nm. Gold was worst, dropping off sharply below 650nm. These are all for first surface mirrors, since as others have pointed out normal glass absorbs UV. Their UV lenses are made from "fused silica". They used to sell a white reflective coating paint, but I couldn't find it in their catalog. That would be your best solution, since it would diffuse as well as reflecting it. I think you would have too many hot spots with a mirror. If you want to go the mirror route, get a standard first surface mirror and don't worry about exotic coatings. I bought an 8x10" piece once from a glass store, they sold it for people who build kaleidoscopes (so it was cheaper than a "scientific" mirror :-)