tigerman,
Depending on how you apply the pastels, you can achieve the desired result and still be able to
carefully dust them if you don't keep them in a case. I use a stiff bristled brush to kind of grind the powder in where I want it and then remove the excess with a soft sable brush. The artist pastels are actually very fine oil pigment instead of true chalk and when you do this, it will latch on to the paint/lacquer surface pretty effectively without needing a seal. Of course, if you get too agressive and use something like compressed air (which is what my wife did, my fault not hers as she'd seen me do that before with other non-pastel treated models...
) the pastel will not survive.
An alternative is to use a weathering powder with a bonding agent such as MIG powders or MMP powders. I've never used MIG, but the MMP have an acrylic bonding agent as well as the pigment and if applied wet with either alcohol, water, or regular thinner depending on the surface, it is workable almost like filling putty and will stay on without any kind of sealing required.
Rather than spend money on individual cases, I bought a nice acrylic wall case online and use that to store my more delicate items. http://store.casesforcollectibles.com has a nice array of ready order cases and will also take custom orders. Very quick response and very friendly to deal with.