Contact the local beer wholesalers to determine if they offer this service to the retailers as an added value free service. If so, it would be difficult to convince to retailer to pay for a service that they receive for free. A possibility would be to have the wholesaler to outsource this service to you. To be profitable, you would need to convince the majority of the wholesalers to use your services so that you can clean most if not all of the lines at the outlet.
Check out these links for
line cleaning info and the
Micro Matic Dispense Institute for additional info.