Hi Mark,
Thanks again for catching this — you were right, the closed test had accidentally been restricted to Japan-only in the country availability settings. I've corrected it to be available worldwide, and the change is currently going through Google's review, so it may take a few hours to fully take effect.
Could you try the opt-in link again later today? If it still doesn't work after a few hours, just let me know and I'll dig further.
Really sorry for the inconvenience, and thanks for your patience!
Regards