There's of course an interesting version control problem when statutes get amended--how do you merge your annotations which are anchored to the old text into the new text?
One big pitfall I've noticed with taming regulatory data is that often each agency has its data siloed differently. Regulations.gov has been great for harmonizing many, many agencies, but here's a list of all the agencies that are currently "non-participating": http://www.regulations.gov/docs/Non_Participating_Agencies.p.... This means you'd need a parser/scraper for each agency, assuming its rulemaking data is even made available in a sane format (like a spreadsheet).
Anyways, best of luck. Successfully wrangling legal data seems more difficult than building the Hyperloop.
Definitions are not as uniform as citations, but they are still pretty formulaic. "_________ shall mean ________" etc.