Discus being what they are, I'd go with assassin snails.Only had clowns in the past are the any type of loach that eats snails but isn't as agro as the clowns. Mellow loach?
As others have suggested, it is quite likely that most loaches would be too active and tend to display what I can best describe as 'pushy' behavior, which could stress discus. I know that Clowns often do that.
The only loach that I'm aware of, that I believe should not pose a problem for discus is the Kuhli loach, but they wouldn't likely help with snail management.