It probably makes more sense to just support remote-unlock and remote-password-change quickly/easily, after the fact. I wouldn't want to depend on duress passwords except if they worked exactly identically and ALSO sent a silent alarm.

For post-contact response, maybe even an IVR system you can call (from a public phone or something) and give an audio or DTMF password.

(I'd also like to get my phone droned after I move out of danger close radius, or the civilized city equivalent (police/bike gang/etc. show up at geolocation))