- Have bitcoin wallet give a warning when you import a backup. That you must backup again to have all your pools
- Have the imported backup be the default pool.
Just to clarify, Blockchain.info/wallet does not use any key pool like the Bitcoin-Qt client does.
If you are using Quick Send, then the change goes to an existing address that should already have been in the backup if you described the sequence of events correctly.
If instead you used Custom transaction and chose "New Address" for change, then yes -- you need to make a new backup after every one of those transactions. Blockchain.info/wallet has a configuration setting for your account to allow "Automatic Email Backups" to be sent.[Edit: The reported issue was with Bitcoin Wallet for Android by Andreas Schildbach, not Blockchain.info wallet, ... somehow I mixed the two up and gave a response that has nothing to do with the question asked, sorry.]