Sadly, no built-in encryption == no cloud backups are recommended.
Do note that files are not stored encrypted, so you should really not do this with rented space (”cloud storage”).
Why so? Nobody bought of making a password protection option with AES256?
I don't think this should be specified as a BOLT because that's supposed to be related to protocol-related things. Given that there's no Bitcoin Core of Lightning network, the best way forward is to take a wallet such as Electrum and implement the backup encryption there a just reuse the wallet password for encrypting the backups. Then if other people think that's a good idea, they'll copy the design.