I'm actually with saddlebag on that one. Could be too much of something. On the other hand, since you changed nutrition she stopped itching except on belly and chest. And she was itchy everywhere with the old feed.
Checked my smart book about vitamin A. 30IU/kg daily is maintenance, and foals fed 20000IU/kg developed scaly skin...it says it needs to be supplemented when hay quality is poor.
Maybe FeedXL is called for, to see what is too much and what not enough. I use Omega Horseshine, as you know, and am thoroughly happy with it, it helped my bug allergic Snipper a great deal, apart from giving her a glass-like shine. Haven't tried smartpac, so can't comment on that.
I'm not using RB, because it's hard to find around here. I feed alfalfa/timothy pellets, soaked, the omega, a tiny(really) sprinkle of strategy healthy edge(the QH gets about a pound, blimpy Arab the sprinkle for taste, and half ration of a vit/min supplement. Horses do just fine on it and I stopped looking for RB.