If you can, hill work with lots of long and low has helped me a ton!
Lots of stretch work on the lunge line! Make sure they have a good forward pace and that the power comes from the hind end
Yes, that is what I have to do with my rescued mare, is working with her over poles and going up and down hills to build up her topline. Feeding isn't going to solve the problem. We have to help them build it up by making them work their muscles. Just like humans horses have to work out too to get into shape.
Be careful with the pessoa, it is a very demanding exercise for the horses, maybe just use it once a week?
Also to build the top line, the best exercise to me is hill work, with a good walk and hindlegs engagement and the horse's head low.
Pole work is another good way too. To maintain the good shape of the horse's back, ride him with his head in the tight position and his withers high while keeping a good impulsion.
Good luck :)