This is where you need to separate the data from the encryption, and where external hardware based authentication becomes useful. If you have a hardware authentication you can destroy the device without losing data, and depending on the ease of getting a new device hold up the ability to access said data. There are all sorts of things you could look at wrt securing/destroying the device, GPS location etc.

If you were to have the devices be stored / issued from somewhere outwith the jurisdiction of your state you might even have something that could stand up to the law. But then you have to worry about someone tampering with the devices - but by that point you're up against intelligence agencies and realistically you can't do anything to stop them - after all your device is Tempest secured, air gapped from the internet etc etc.