I thought some people might be interested. I had an old CO2 tank with a regulator and needle valve left over from my aquarium days. I put a couple of potted phals in a large kitchen trash bag and used a twist-tie to seal the neck of the bag around the 1/2" hose from the tank. I regulated the flow to be just enough to keep the bag inflated. After 4 hours I removed the plants. The visible scale insects seemed to be quite dead, which I judged by the fact that they were no longer firmly attached to the leaf.
Have I finally found a way to win this battle? I sure hope so.