The actual problem is one of power and intention. Yes, version control would make earmarks obvious. Yes, it would make tracking contributors (lobbies) easier. Yes, it would make tailoring tried and trusted legal documents easy.
All of these are reasons why version control will never be applied to the law. We want these things, but we are not the customer. The real customer is actively trying to prevent these things from ever happening.
The reality is that these tools are probably already being used for these exact purposes - in private, and for personal gain.
Of course we won't see Congress outlaw lobbying, but there's definitely an opportunity today to try some of these concepts in an organization, community, or even willing local government.
The problem is that the government is too big and has too much power, so the rewards for lobbying are so great.