I've decided to give one of the herbal supplements a try. The one I'll be trying is Mare Magic, which is mainly raspberry leaves. Most horse apparently like eating it, and it doesn't have any side effects or risks that I've been able to find. Many people seem to have found it helpful with their mares' dispositions and behaviors, although in many cases it takes a while to see effects. The magnesium/chromium/selenium/vitamin E supplement Dawn's on for her insulin resistance may also be having some calming effect - magnesium is often a component of calming supplements.
I've ordered it, and will do an introductory loading dose and then continue it for a while to see how she does. Now that we're heading into fall, her heats should be less intense in any event. The real proof will be in the spring.