Thanks for your reply!
Yes, we want those posts to be publicly available. Here is the use case:
We expect users to upload “projects”, i.e. posts with special attachments (some sandboxed code that can be run in the browser). We know some users have already asked for collaboration, i.e. multiple users can edit the same “project” asynchronously. So this is different from Wiki, which everyone (within a group) can edit.