My opinion, as a web dev who strives for semantic and accessible document structure, is that you cannot put such a tag ahead of the title of the post. The post is the thing being cross posted. Not the community, not the user, not the whole app, etc.
So now it’s just a matter of how important the declaration of cross posting is. Is it more important that the (preview) body copy? Maybe to some communities but definitely not to others.
Goddamn when does the metal start?