if the iPhone reboots, the key code must be entered as touchID does not work. Passwords are still protected by the 4th amendment in the US, right?
Beautiful! The perfect reaction to hearing of any such dragnet-style announcement should be to power-down you iPhone.
Alternatively, download your local ACLU's App. "CA Justice", for example, will record everything. If anyone attempts to operate the phone, it will automatically upload the collected A/V to an ACLU server, and provide you with a reference # in case you need to find it later. As in, "Officer, I do not give you permission to operate my phone. If you do, it will automatically upload our entire conversation to an ACLU server...", which is out of my or your control, and cannot be erased. Then, the first chance that you get, you either power it down, or if they confiscate it, use iCloud's "I lost my iPhone" to nuke it remotely.
Or just keep some sandpaper handy at all times...