Without any major changes, struct tags cannot solve the majority of the problems, because they do not propagate. The v1 MarshalJSON/UnmarshalJSON interfaces are too simplistic. At the very least, an alternative pair of interfaces should exist, which take options/flags/tags/whatever in the parameters. However, I agree this doesn't need to be in an entirely new package. It's not even hard to favor one interface over another:

  switch ifc := val.(type) {
    case MarshalJSONV2: // new interface, always used if present
    case MarshalJSON:   // old interface, fallback
    default:            // type isn't self-marshaling
  }