Oleg Afonin, writing for the ElcomSoft blog:
What we discovered is that iOS will reset the USB Restrictive Mode
countdown timer even if one connects the iPhone to an untrusted
USB accessory, one that has never been paired to the iPhone before
(well, in fact the accessories do not require pairing at all). In
other words, once the police officer seizes an iPhone, he or she
would need to immediately connect that iPhone to a compatible USB
accessory to prevent USB Restricted Mode lock after one hour.
Importantly, this only helps if the iPhone has still not entered
USB Restricted Mode.
Most (if not all) USB accessories fit the purpose — for example,
Lightning to USB 3 Camera Adapter from Apple.
They think this might be tricky for Apple to fix:
Can Apple change it in future versions of iOS? To us, it seems
highly unlikely simply because of the humongous amount of MFi
devices that aren’t designed to support such a change.
Theoretically, iOS could remember which devices were connected to
the iPhone, and only allow those accessories to establish
connectivity without requiring an unlock — but that’s about all
we can think of.