Q: Can't this be done on Wikisource?
A: No, it cannot. Works that are constantly evolving (like most laws) are out of the project's scope. However, this does not mean there isn't room for the two projects to work together.
Q: Okay, what about Wikidata?
A: Wikibase (the extension that powers Wikidata) is limited by its ability to store long strings of rich text. It just wasn't meant for that kind of use case. WikiLaw will likely utilize Wikidata to some extent, but rich text is going to be needed to be stored on its own wiki.