Turning a spare phone into a Seedsigner: Stateless signing device from old hardware.

Is it possible to turn an old phone into a seedsigner? Most people turn phones into hardware wallets but I'm not confortable storing my private keys in the phone's SSD, even if doubly encrypted (phone encryption + wallet encryption).

One big advantage is the stealth factor. You walk through customs and you put a huge target on your head if they recognize your Seedsigner. Phones on the other hand are everyday carry for 99.9% of people.

Is there a way to use the phone solely as a signing device where the private keys are held only temporarily (either RAM or wiped clean after use)?

Of course all conectivity other than the camera would be disabled.