Well Steve,
The only things I can think of may compromise the value of the beads and darken them. David uses penetration epoxy on wooden boats for the stability aspects. And apparently such stuff is used by some museums to stablilize fractured glass. What about spray-on polyurethane type stuff? I used to use Krylon matte-finish to keep patina-green metal from "shedding" with minimal darkening effect. ALWAYS use such stuff with maximum ventilation or even a respirator. Beware of ingredients such as Xylene and Tuluolene - anything with an "ene" - one spray can even had a warning that pregnant women should not use it, as it "may cause miscarriage". Seriously bad stuff.