Thanks for the quick reply. I was thinking the preview would depend on the Discourse topic/comment creator’s access, not the Discourse viewer.
I was thinking about the preview as a snapshot that gets created and tied to the post as flat HTML, not a dynamic widget - I hadn’t even considered that as a possibility!
If I understand you correctly: this would possible with customization (via script) on the Discourse side, and/or custom access logic on the Drupal side?