> or maybe this functionality is available already

Basically already exists depending on specific trade offs and risk profile.

You already can encrypt your data and store the encryption key offsite. But then you couldn’t use your phone during travel, if you toss the key locally.

You can encrypt the data at rest and leave the decryption key in RAM and just turn off your phone. But they can still take the phone and copy the encrypted data, if they think they’ll get the key later.

My understanding is that this individual would t want the government to access the encrypted data either.

I'm talking specifically about the graphene OS ability for that approach, not the ability to add an external key to some generalized encryption. The threat model here is that the traveler was required to provide a passcode unlocking a key they had with them on their phone. If that threat is not there, then this bypasses problem.