I can't remember where, but I remember some other Meta post on some science Stack Exchange site discussing crossposting, and some users there approve.
Instead of trying to find a list of venues it might be possible to post a particular question to and stay technically within their rules, I think it'd be better to carefully look at what's on-topic, both in letter and spirit, on a particular site you are considering posting to. And not to keep trying to find new angles on a question that has basically been answered.
Theoretical Computer Science Stack Exchange has a similar policy (cross-posts are allowed after 7 days); see this answer to their FAQ. Since it mentions 'other Stack Exchange sites', it doesn't seem to be a mutual agreement.