I also didn't know about the parasite risk, even from store-bought earth worms, until recently. Your advice is sound, but I would offer a friendly challenge to a couple of points. First, nutritionally they are fantastic. Rich in calcium, high in protein and low in fat. So, there is certainly a reward. Plus, they are great as dusting critters with sticky coating and large surface area. Is it enough to offset the risk? I would say probably not...
Two, I believe Vailed Chameleons do in fact eat critters off the ground from time to time, especially after rainfall. In fact, someone here on the forum stated they have personally observed this behavior in the wild. They dwell in trees and shrubs. Plus, they are ambush predators. Opportunistic in nature, so I don't think we can completely rule out the possibility they eat earthworms in nature.
I agree with the OP that if there was a way to ensure parasite free earthworms, they would be great addition to the feeder staple.