Yeah extra backups isn't something I've specifically thought of but it's very in line with where we need to go if Hive starts gaining traction and we can afford to spread the wealth around a bit more for extra network security.
As we see on Steem, the backup witness can really stick it to the network when they fuck up, because the backup witness is including blocks that contain frozen account transactions, this causes the chain to become unstable because the top 20 have to roll it back... this is exactly what they deserve to be honest.