I can't help with the show part, but if you add some fat to the diet, it will help or get rid of the dandriff. May take a few weeks to show up though. I like BOSS (black oil sunflower seeds) 1/2 cup a day or Canola Oil 1/2 to 1 cup a day. Also rice bran is high in fat 1/2 to 1 cup a day. Add to that a ton of brushing. I also have found that when I don't shampoo a lot the oil sticks around longer, and there is less dandriff. I hose off, but don't shampoo. Of course I don't show either... So, that is probably not an option for you.
Dont ask me why this works, but dilute a little unscented listerene into your bath water. This is a tip I got from the pony club manual and they are sticklers for having everything work...so it should be a good option.