Hi Nick, regarding your questions:
You are very welcome to sign up to the social media mailing list, which is where we currently post draft social media messages for review, often in parallel with Social media/Calendar. Feel free to post suggestions yourself, or weigh in on other people's proposed messages (in the WMF social media team, we generally run on a four-eyes principle). It's also a good place for more general discussion about social media related to Wikimedia projects.
Oh, and it seems we forgot to link @wikivoyage in that anniversary tweet - sorry about that, but there will be other opportunities!