Did you look around to see if there was anyone to take advantage of such a service before you opened up? If not, ask around. You may be surprised, or you may indeed find that there's not much of a market in your area.
Just realize what you're getting into. Horse boarding is already not a particularly lucrative business, and even if they are paying vet and feed you will likely still end up in the hole. What about farrier, hay, worming, etc? Not trying to discourage you from helping our service people, but just make sure you know what you're getting into!