Have it as a second factor. Just make it harder for police to confiscate devices. U2F keys are great but they can be confiscated too along with your password. Fingerprint or face is possible to copy but much harder. Most police don't walk around with 3D face scanners.
Also make software self-destructing with a warning, i.e. if the user chooses it at installation time, all data will be destroyed automatically by the OS if they move the device off-premises. Make the setting unchangeable after installation time.
Police won't want to destroy evidence, so they'll have no choice but to leave it on premises.
I'm not trying to enable criminals, but rather enable whistleblowers and to not succumb to unreasonable new laws, and keep unethical searches for bad reasons in check.
Also make software self-destructing with a warning, i.e. if the user chooses it at installation time, all data will be destroyed automatically by the OS if they move the device off-premises. Make the setting unchangeable after installation time.
Police won't want to destroy evidence, so they'll have no choice but to leave it on premises.
I'm not trying to enable criminals, but rather enable whistleblowers and to not succumb to unreasonable new laws, and keep unethical searches for bad reasons in check.