I've updated the scheme:

1. It describes data format and crypto in full detail.
2. Key derivation and signing is simpler (HMACs instead of ECDSA and BIP32).
3. Merkle tree support to allow efficient periodical "proof of storage" requests.
4. Method to efficiently timestamp backups on the blockchain so you know which one is the latest one.
5. Method to do incremental backups if they unusually large.

Let me know what you think:
https://github.com/oleganza/bitcoin-papers/blob/master/AutomaticEncryptedWalletBackups.md

Thanks!