I would check with Thompsons enamel company for the specs on the amount of lead in their vintage enamels. As for the Open Studios event, I would not use LEADED enamels in demonstrations if you are sifting, but there is no problem demonstrating wet packing - however, I would not waste my precious vintage enamels in demonstrations if that is what you are participating in, because once they are gone, they are GONE! and some of the colors are fabulous - I would use unleaded and make it very clear to all that are participating that you are using UNLEADED glass. Most all workshops and learning centers only use and supply unleaded enamels for health purposes. People get very touchy about Lead, altho I have used leaded all my working life- been tested and the labs come out clean - its all in how you process and use leaded enamels - I don't sift much, so the airborne issue with lead is not a problem for me.
goodluck and have some fun!
You might find this article helpful. It goes over the various classifications of lead content and what requirements are in California. It also clearly points out the difference between metallic lead and lead oxide and how lead oxide after heating in glass is not an issue.