I was going to suggest apple sauce or apple cider vinegar. Start with just 1/4 cup of apple cider vinegar for 3-5 days. Once she's eating it well, you can increase it to 1/2 cup, or leave it at 1/4 cup. ACV will cover the taste of almost anything!
When you run out of rice bran, you might consider flax. I haven't had any horses turn their nose up at ground flax. Horsetech.com will send you free small samples to see if your horse likes it, and they can add a flavoring if she doesn't (their web site prices include shipping). I just get cheap no-name ground brown flax from my feed store, as all of my horses like it just fine.