I recommend you make a backup to your laptop, which you then encrypt manually. That way the trust model is: you trust yourself. Then you can do whatever you want with the encrypted file. Apple's iCloud is perfectly fine at this point.
The real challenge is to find a way to restore that backup, because you have to be on a computer you trust. If you decrypt the backup on a "loaner" laptop, your security is broken.
If you decrypt the backup on your personal laptop but the laptop has a hidden keylogger installed by the TSA or TAO, your security is broken.
It would be necessary to backup the phone on the _phone_ _itself_. Then manually encrypt the file (easy to do). Then upload to iCloud. At this time, no such app exists for iOS.
Since you plan to restore the backup to the phone anyway, it's no problem to decrypt a file on the phone before using it for the restore.
