If only these phones could have a feature where you can enter a mode to encrypt segments of data with different encryption keys, all able to be unlocked still by one single master encryption key that you could also enter at any time.

Then when in travel, and about to pass a border, you enter that selective mode. You can enter a different password as the proxy to unlock the phone in this bare minimal data mode. The agents can access it, etc but wont have all the data. That data shouldn't even be visible in the OS, and if they try to copy the hard drive they will get encrypted data in the other blocks.

You still legally comply with orders and unlock the phone but the other partitions don't unlock/decrypt unless you specifically unlock them.