On other related threads, a couple of us discoved this paper from a year ago, with strong experimental evidence for the efficacy of sulfur compared to other non-toxic mite treatments: http://www.thepoultrysite.com/articles/2470/novel-control-of-fowl-mites. Some other sources I've read indicate that ingested sulfur has no effect, but some people still claim that it works against mites when mixed into chicken feed at something like 0.2%. Whether that's because it actually works from inside or because some of it gets everywhere when the birds eat seems to be an open question.
Anyway, to summarize, elemental sulfur has been known for a long time (centuries) to be effective against mites, fungi, and some insects, while having toxicity comparable to table salt. We got mites on some chickens recently, and started with diatomaceous earth, but after learning more I ordered 10 pounds of sulfur, probably several years' supply.
Bottom line: there's probably not much point in getting your chickens to eat sulfur, although it's not going to hurt them if they do. Following the experiment results cited above, mixing sulfur powder at 10% by weight into some of your dust bathing areas, and hand-dusting infested birds on occasion, should do the trick. According to that paper, sulfur is much more effective than diatomaceous earth, with longer lasting effects that also extend to chickens that don't dust bathe but live with those that do! Sulfur is cheap, too, esp. considering that you need to use far less of it.
Note to Erica: I agree about avoiding the more toxic insecticides/acaricides, regardless of whether they're from natural sources (some of the most toxic substances ever discovered are from natural sources - ricin, for example, or saxitoxin). But it's generally a mistake to conclude things like "garlic works by being high in sulfur." This is a common error among many who comment or question on BYC. What we're talking about here, elemental sulfur, doesn't necessarily have anything in common with compounds that contain sulfur. There are thousands of such compounds, ranging from essential nutrients and biological building blocks, to lethal poisons. As an analogy, we could point to methanol or organophosphates, or any of the countless deadly poisons and carcinogens that contain carbon, and say "We should never eat or use anything with carbon in it." But that would rule out sugars, starches, proteins, and every single food item that I can think of offhand.