Sweet action! Depending on how far you want to go with the Bitbucket integration, you could potentially inject warnings on the commit, branch, and pull request pages using the Bitbucket Connect framework.
One of our engineers wrote an add-on recently for enforcing some less automated aspects of CONTRIBUTING.md that you might be interested in too: https://blog.bitbucket.org/2016/01/26/pull-request-guideline...
