I think the future defense against this sort of thing is more situational awareness on the part of the phone. You won't just have to fool the biometrics, you'll have to do so without making the phone suspicious at any point. My guess is that phones already have enough sensory data to pretty reliably distinguish everyday usage from being stolen or confiscated, and it's "just" a matter of fitting a model. Nor does the phone have to ignore what happens to it after it's unlocked.

No one tries this sort of thing on people or animals, and it's because they don't shut their perceptions off at all times except for a half second when they are authenticating someone.