Indeed, mirroring solves the "discoverability" part of the problem. We had that set up. But it introduces new problems.
The "Participation" becomes more complex: where do people post, update and search for issues? Where to update the wiki? How&where to update a PR? How to authorize push acces, etc?
We'll still need a canonical, clearly marketed, "single source of truth".
Should that be github? Or should everyone be directed to the selfhosted version, away from Github?