I think a camera with a view of your device and the windshield or dash would probably suffice in most cases.

But, you may be right. Someone else said they were affirmative defenses, like when you are charged with driving without a license the police don't have to prove there was no emergency, but if the defense proves the emergency you are acquitted. Unfortunately while this law is explicit about that example, it fails to say the texting exceptions are affirmative defenses, so I don't know.