About flax - I just buy the whole seeds in bulk at my local grocery store ($.60/lb) and they get digested just fine. It's MUCH cheaper to do it that way and, ime, nutritional value is absorbed whether they're whole, ground, or oil.
I know nothing about mini's and I've never fed a lactating mare (aka, I'm sticking my nose where it doesn't belong so feel free to disregard everything I say,
) but since you cannot feel ribs, I would not think she really needs to put on a lot of weight. Unless mini's are vastly different than big horses in that way, you always want to be able to feel some rib... I would bet it's just a whole bunch of muscle-loss that you're seeing and not so much weight loss - before this baby, you had her at a darn high level of fitness! And the only thing that's going to put on muscle is proper work, not feed.
Anyway, if you're REALLY concerned, I might give her even less than half a pound of rice bran/day (also, the "worrisome" thing about RB is the Ca:P ratio, for the record, :) ). I mean, I had Lady on one pound of it a day and she put on over 50lbs in one week! And she was a 14hh, 850lb, neglect case!
Of course, Lady wasn't nursing but, due to that, I'm inclined to think even half a pound/day might be too much for Sour, given that she's not actually ribby or anything....
But, I know absolute nothing about mini's so correct me if I'm wrong! :)