Thank you everyone for your replies.
I completely agree that this poor dog was suffering from untreated thyroid issues when I found him, and since we estimate his age at around 5 I assume that the loss of thyroid function is due to autoimmunity rather than age (I guess we'd need a biopsy to know for sure). In which case that gland is toast, and we'll need to supplement. Just need to get the right dose - it's too high now (got results today) so we'll move it down and check again in a few weeks when the steroids are all gone.
In regards to diet/cleansing/yeast elimination, I appreciate the suggestions, but I can't find any evidence that a detox is going to work. I realize people have tried this, and had success, but the scientist in me can't justify it without some precedent in published peer-reviewed literature.
Plus a Candida detox might not be as useful against Malassezia yeast? I was under the impression that oily blackened skin is usually due to M. pachydermis, though candida species have also been cultured from the skin of dogs. And since Malassezia are lipid-eaters, I don't think eliminating sugar will help much there.
I wasn't aware of colostrum as an anti-inflammatory, and maybe a big dose of Proline-rich-peptides (not "praline-rich peptides", as written in the linked article, though those do sound yummy
) might be immuno-regulatory. Interestingly, that compound is now in Phase II trial for Alzheimer's treatment, but I digress....
I think we're in for a long battle with immune hyper-reactivity in this guy. Hopefully we can figure out what's triggering his itchy episodes and head it off in the future. It doesn't help that we live in Houston - no true seasons, allergens everywhere, and humid!