There are already people intentionally buying beef straight from producers, to ensure that it's "grass fed". If warming enthusiasts cared as much about this issue as these paleo dieters do about avoiding flavorful beef, they could do the same.

As you say, the effect would be small, but why would that stop anyone from doing it?

Oh I reread the thread and saw you were only addressing whether there is an immediate incentive. Yes, you're right, there certainly would be one, and it probably could be organic/paleo level successful. Probably doing it privately successfully would help rally support for an eventual law.