Well, if you've got a magic secret bullet for derailing the farm lobby, please get down to the Republican Congressional leadership's offices pronto and help them stop the vile travesty of a farm bill that's about to be passed.
Short of that, it's really hard--nearly impossible--to alter programs in such a way as to take away benefits from a key constituency.
So I propose a two step program for reforming WIC:
1) Build a new program that actually does something useful. Let it accumulate bureaucratic power, hopefully under another agency than the USDA.
2) Have it assimilate WIC. It's well known that a bureaucracy can only be killed by another, more powerful bureaucracy.
But I doubt WIC itself will ever be any good. It's been thoroughly captured by the farm lobby.