You really wouldnt need a law passed. Any slaughter house that opens will need a business licenses. During the application process it is easy to "Ask" a business to do something as part of the aproval process. Usually businesses are happy to oblige something like that that is simple and cheap. When this process takes place horse owners will be way better served asking for mandatory scanning and owner notification, than screaming and crying about how bad it is to slaughter horses.
Remember there is and never has been a US ban on raising and slaughtering horses for food. This is a myth that has been perpetuated by uninformed people.
There was a budget cut at the USDA, Since no one in the US was eating horse meat the argument was made why are US taxpayers paying for horse meat inspections ? The funding for the inspections simply stopped. The industry could have paid for inspectors and passed the costs on to consumers, but there simply wasnt a market. Slaughter houses went to mexico for the same reason Wirlpool, Hershey, and others went to mexico. NAFTA made it easy, and cheap labor, no oversight, and no environmental laws. I will be extremely surprised if a plant opens here. Why pay US wages, and deal with US laws when you can do it in mexico ? I guess time may prove me wrong, but I dont think much will change.