This doesn't sound plausible in the slightest. The only persistent connection Apple has that I can think of to implement such a concept is for push notificatio

This doesn't sound plausible in the slightest. The only persistent connection Apple has that I can think of to implement such a concept is for push notifications. Which would be a massive security hole if a HTTP response to that daemon was capable of bypassing the lock screen, secure enclave etc. And the logical question is if they had such a system why would they bother triggering an iCloud Backup when they could ask the device to specifically hand over certain information e.g. Messages. Which at least could be done quietly over Cellular.