This will not be possible, as the forum's authentication is not exposed via an API, and it would not be practical to do so. We would likely need to use something like Okta token authentication, and the forum is not set up for that.
If that is truly the case, then there are no issues with your site accepting uploads and listing them however you wish.
You are thinking too high tech. This is what I am thinking:
When a user creates a new account, I'll go to OF and search for that user there. If I find a match, I'll ask them to post in a thread on OF with the user. Their new message in accordance with my instructions will establish identity. If they don't post, their account will be deleted in like 48 hours.