I don't think that getting a prosecution here and there would be that difficult. You won't get what you don't ask for.
You don't need many to send a message. I suspect that any such prosecutions would be widely reported. Not in the MSM perhaps, but certainly in the alternative media.
I encourage you in your letter writing efforts. From the RKBA world I know that it does work.
I'm certainly going to try. I'm going to start with local representatives and work my way up.