I've been looking at rebuild as well for my horse so if you can update on how it goes for your horse that would be awesome.
I use nograin gold or blue, depending what the produce has at the time and he gains a little weight with that and pollard.
Copra is also a good one for weight gain, but my horse doesnt like it :(
I've had some horses respond well on full fat soya bean meal as well so you might want to give that a go as well.
Try Prydes Biomare. That makes most horses put on weight. I would also add Rice Bran Oil which is usually readily available at Woolworths. On special it is $10 for 4L tho can also be $13 for 4L normal price is $20.
My OTTB gets Barley, Copra, Show torque and lucerne chaff with a good cup of RBO and ready access to OK pasture and a decent grassy round bale. He has lovely dapples tho is still a little light on