I think one possible way of fixing that counter-counter-PAC problem is this: Instead of focusing on the politicians themselves, trying to convince them to only accept money from disclosed sources, one could focus on convincing the existing corporations/PACs to only donate to politicians that disclose all their sources. That way you end up getting the politicians on board, and you're not at an arms race with anyone. Instead you're using the money from whoever you manage to get on your side to your cause's favor.