But this means there needs to be someone monitoring posts, not just every five minutes, but every five minutes, for every server.
This is completely untenable. An off-instance sub might not even have enough subscribers on that other server, to count on both hands. Yet someone has to mod it? For small communities, there might just be one or two subscribers to it per server.
You're giving examples with massive usercounts, which wouldn't work, due to that massive usercount. But low usercount examples also don't work, due to the low usercount.