Sorry for the delay, things got real whacky.
Well the way I’d hope to do it is to have them see an error like “the beta has now ended, join the active discussions on the live build here [link]” or something, redirecting to a regular account signup. Just some intervention that lets you tell someone the thing they clicked is no longer valid, but they can still “take part” in talks about the game, basically what you suggested in the /admin/customize/text section.
The forum is otherwise open, so we wanted to transition players to the public categories by giving them a graceful transition via their expired invitations to those public boards. Right now it just puts players in a category that’s no longer visible, so a less than ideal first journey for that user.
We’ve not really had much pushback or confusion as it goes, so it turned out to not be a big problem. But I know for next time how to do that - so thank you hugely.