@Leo,
Is the "Account Linking" part of the forum you built Open Source, or built on top of an existing open source plugin that you can refer me to?
I'm pretty familiar with Discourse, so I assume you've probably got a custom field on the user that stores their registered serial number, and a cron job that imports your valid serial numbers every few hours to make sure you're matching against a valid list. Perhaps you've got some anti counterfeiting logic to notify and/or block in cases where a serial numbers has been leaked publicly or where used repeatedly.
In addition to the custom field with the actual serial number, you probably have a second custom field that directly reports the linked / validated status of the serial number so that you're not comparing it each time, and can manually over-ride it if necessary.
Obviously it's not a super complex thing compared to Opus itself, but there's almost always more complexity to things that you estimate at first glance...
I would be most interested to hear what you would do differently if you were to implement this feature all over again from scratch?
Very grateful for any information you can share!
You're welcome to convert this to a PM if needed.