You know, since we are entering into a lot of legal weirdness here, how about this:

You give the cops TWO passwords. You tell them one is the duress password, and the other is the password that will unlock the phone.

Desperate and without a warrant, they try one. Wrong try! Phone is wiped.

In reality, both are duress passwords but they would have to prove that. (GoS only currently supports one duress password)

Yes, it's ridiculous, but everything about this is ridiculous.