Endiku- Great job with your filly! (first off)
It's been my experience that the average horse (not under nourished, not growing, not in hard work, that is) that is fed with any too-high a protein diet (any number of supplements, not just rice bran) can start to exhibit problems that could otherwise be averted (too much of anything isn't good). I tend to be more cautious with feeds and supplements (I have a less-is-more-policy). So that wasn't directly toward rice bran, but just any any hi-protein type feed/supplement for long term use without a specific plan.
Given that we were given an open question, not really specific to any particular circumstance, I was just giving a general answer (opinion). I tend to err on the side of caution when it comes to extra feeds and such- but my philosophy is to try to get as much from hay/pasture as you can and supplement from there.
I do think it's a great option to help malnourished horses gain good weight/muscle, but I personally think it's too much for the average horse. (It's got a lot more fat, and alot more protein than beet pulp).
But, I know that everyone tends to have different ideas and thoughts on things- and there are so many circumstances we could probably write a 3 part novel on feeding philosophies, lol.