This can be bypassed by putting the phone in DFU mode.

Probably the only hope is jailbreaking.

Jailbreaking a locked, inaccessible iphone?

Keep in mind that everyone else is usually unaware (by design) of what all the intelligence agencies can do, but I doubt they would help in this scenario even if they could.

On the other hand, if this happens to a far more important person...

Jailbreaking is dead.