I'm the lady who was given depakote by my detox facility for 3mg a day Xanax addiction. Since that time, I have also looked for more information on the web about it, and am regularaly shocked to not find much. Perhaps it isn't very well known yet. But there's no reason why a shrink couldn't prescribe it to a patient along with the Xanax to help the person get off. My instinct would be to call my detox facility, Silver Hill Hospital in New Canaan Ct and ask for more info - or have the doctor do this. Perhaps facilities more local to you and your brother would also have some information.
I have no idea how depakote works in patients on very high doses of Xanax like your brother. 20-30 mg is certainly a lot.
When I checked into Silver Hill, I was surprised that they didn't taper me at all. It was "Hello, here's some depakote, and no more Xanax for you". Based on my prior experiences with trying to taper, I was prepared for the worst, so it was a major shock to find out how much easier it was on Xanax. I was even able to sleep at night! As I've said before, I felt like a had the flu, but more of a mild flu. After 3 days, it just switched over to mild nausea, which lasted about 3 weeks. And in weeks 3-6, I felt OK, but a little weird. After week 6, I was pretty much back to my old self. They kept me on depakote for 6 weeks total.
What I was so thankful for, was the depakote kept me from feeling like I was going to combust at the seams. I suppose that's related to it being an anti-seizure medecine.
Perhaps it isn't highly publicized if it is still in relatively new use in detox clinics. I don't really know. I wish I had asked more about it when I was there.
Good luck! I just hate seeing people struggle through the "Ashton" taper when there is an alternative to try before going through the Ashton method.