Doesn’t the Forumverse already do this by default?

Lemm.ee was closed a long time ago, but all of its communities still kind of exist, because every instance that had federated with its communities, have full copies of those communities. For example:

• https://piefed.social/c/cartographyanarchy@lemm.ee
• https://lemmy.world/c/cartographyanarchy@lemm.ee
• https://slrpnk.net/c/cartographyanarchy@lemm.ee

Those three are the same community, but also not. Now that lemm.ee is not around to do the federation for that community, no data gets transmitted between the three versions of that community I linked above. Just look at the number of upvotes: They are different behind each link, because there’s nothing telling lemmy.world that someone on slrpnk.net had upvoted something there. You could even add new posts in any lemm.ee communities that are not locked! Those posts would only be visible for people on your own instance, though.

In the case of a permanently dead instance this is something of a problem, actually, because people might not understand that they are posting in a community that will never again federate.
But, for example in the case of the downtime of slrpnk.net, people were still able to use the communities. They were able to upvote and downvote, they were able to write comments, and they were able to make new posts.

All that content was stuck in each individual instance, so if I from nord.pub wrote a comment there, a user on lemmy.world would not see it. But, when slrpnk came back up, all instances sent all of the updates to it, and it then federated then onwards to other instances, joining all the added comments into one.

The whole Forumverse functions through mirroring. Each instance has a full copy of any community its users have subscribed to. If the home instance of the community goes down, the copy will continue to function.

So, to write all this in the nutshell: While slrpnk.net was down, people using other instances were still able to use its communities without even noticing the server was down! Therefore, I would argue that what you are asking for, is already taking place. It does not work very well if slrpnk.net ever goes down for good, but as long as it eventually climbs back up after disappearing for a while, this solution works very well.

And also: If lemm.ee somehow ever comes back online, all the upvotes behind those three links will be federated to all other instances, and then any of those three links will show the same amount of upvotes.