There is at the least one 'Aquatred' in the UK that may help you. It is a long trough with a slope each end , a treadmill in the flat bottom, and Jacuzzi type jets in the sides. It supports the horses weight with water while allowing exercise and stimulation with the mill and jets.Cant remember where it went but I helped with the shipping so I know its here somewhere !!
My vets has a full rehabilition spa at their yard (or pampering, if you're feeling generous!) and they charge about the same £25-£35 per 20 mins. I have recently looked into the benefits for Eriks DJD as he currently has to have his chiropractor every 4-6 weeks, to keep him in light work; will let you know if we decide to go for it!