DaveMc1 - yes indeed, you are correct, to a certain extent.
What actually causes the concentration of energy that results in a spark, is sharp edges and corners. So you can even get your sausages to spark, if you slice sharp edges into them.
Metal simply makes things worse by allowing the electrons/energy to flow easier. The electrons concentrate at the sharp corners and spark. This is why spoons are harmless on the MW, they have rounded edges. Forks cause far more problems because of the tangs.
Glitter is just perfect for sparking, with all its sharp edges. The example that I read of glitter causing the problem, has the metallic glitter glued as a decoration around the edge of a paper plate.
Having the glitter suspended in plastisol may well be enough to absorb all the energy and no problem will exist. There is only one way to find out. Just keep your finger on the stop button for the test. All I can do is the research and then pass the information along to you guys. Practical experience will always count for more, and so if you guys say that it works, then that is good enough for me.
Stay safe my friends
Dave