Bran mash is actually really not good for horses. If you want to feed him something mushy, a bit of beet pulp is a better treat. If your horse is getting his teeth done chances are he is going to be sedated for it (I dearly hope at least) and then he really is not going to feel like eating for at least an hour after. After he gets his teeth floated the best thing you can do for him is keep him out of the elements and away from other horses until he perks up, so in a stall or outdoor pen alone in a shed. Plus their mouth really shouldn't be too sore after, at least not enough to deter eating, I just wouldn't put a bit in his mouth for 18-24 hours after.
As far as for horse's diets I recommend feeding mainly forage and a balanced vitamin/mineral supplement. If the horse is in work, or having trouble keeping weight is the only time I would start to feed concentrated feeds, and even then you want to keep the fat content high and the starch content low. The best thing to feed is beet pulp, rice bran or a soybean hull based product. Bran mash has a high starch content along with most to all grain based, wheat based and corn based products.