Note that this won’t include any account based mechanism.
AFAIK out there isn’t any such solution at large. But it can’t be too hard to develop. It is basically just static not-up-to-date-mirror with proper headers for Google.
But what would be the point for that? Quite few don’t use JS and when one is doing that he/she knows that the most of functionalites offered by sites are lost.
Sure, that would be blazing fast and easy to cache. But… why? Discourse isn’t painfully slow. It would be a honeypot for useless seo-bots, but even then those wouldn’t leave ”master-Discourse” alone. It wouldn’t help googlebot or does anything helpful as far as I can understand.
There is one situation where it could be helpful, though. And it is situation when Discourse is down and server/reverse proxy/load balancer knows how to redirect traffic.