I used Triple Crown Senior for the last part of my 25 yr olds life - it made a drastic difference, the only reason that he eventually passed was because his small intestine just wasn't absorbing nutrients anymore.
If you use rice bran, pay attention to the calcium/phosphorus ratio so that you don't get it out of balance. But rice bran is a great idea.
A cup of corn oil works great - instant fat calories, but the main thing I read and agree with is freechoice hay, with alfalfa or beetpulp given as a supplement to that but not as the main dish.
I agree about getting a dental workup done just to be sure, a deworming for tapes, and then don't forget winter blanketing especially during the rainy days - a horse can shiver off all of the calories he ate the day before in just one night of cold weather.
The main thing tho is good quality free choice hay and a vitamin supplement or at the very least, a mineral block. If you horse is healthy on the inside and doesn't have any metabolic issues, hay is the fastest, cheapest and most efficient way to bulk them up. But if there are metabolic issues present and hay isn't enough, your idea of Triple Crown Senior is a great feed choice for the old skinny horses. It worked for my guy - it added years to his life, and he actually got back to his old self and we did some more trail riding and playdays for a long time.