I believe the 2 lb limit has more to do with the calcium:phosphorous imbalance than the fat content. 2 lbs should be plenty for most horses, though (it's about the same amount of fat as 1 cup of oil). The Ca:Ph ratio is less of a problem if the rice bran is fortified with added calcium, but still needs to be paid attention to. If you do find yourself needing to feed more than that, it would be worth trying out FeedXL.com to make sure the diet is well balanced.
The powder (called rice bran meal) can be fed as-is. It's what I feed my horse (since it's the only stabilized rice bran I could find in my area) and he loves it and even licks his feeder clean. I feed 1/4 lb twice daily, mixed in with his Triple Crown 30% Supplement. You can wet it down or add it to a mash if you want- it never hurts to get extra water into your horse
ETA- You should also be sure to add rice bran into the diet slowly, as too much fat too quickly can upset the horse's digestive tract (much like any other major dietary change).