akashic-
I used to race bicycles. When we raced with our disc wheels, we covered the valve stem holes with stickers like these-
10 Disc Wheel Stem Covers for Zipp, HED, Mavic Etc. - eBay (item 370199810777 end time May-13-09 19:03:02 PDT)
of course, any stickers would do. Hope that helps!