My barn feeds a beet pulp/rice bran (4:1 ratio of beet pulp pellets to rice bran pellets, by volume) mash every night, and I've noticed that it does take a little while for new horses to accept it. Whenever we get a new horse in, we just offer them the scoop with a little mash on it through the feed window and let them kind of lip it; we don't put it into their feed buckets at first because it will just sit there and have to get cleaned out before the next feeding. Within a couple weeks they're all just as excited about it as all the other horses and we can give them a normal amount directly in their feed buckets. There are 76 stalls at the barn, and I don't think there's a single horse there who doesn't love mash
Rice bran by itself is great for putting on weight and doesn't need to be soaked. Do make sure you get stabilized rice bran as it goes rancid very quickly. Also try to get fortified if you can find it- this has added calcium to offset the high phosphorous content. Fortified may or may not be important depending on how much you're feeding and the Ca:Ph balance of the rest of your horse's diet.