It is commonplace for a few items of safety equipment to be stored in overhead lockers. Usually those closest to the crew positions. Exempt those bins from any auto-lock system.
The big danger is the delay caused by pax in an emergency retrieving their baggage, with a secondary consideration being the obstructions caused by these bags, especially if abandoned. If you only have a few seconds to get out, then tiny details become important. jb747's example of the
Airtours 737 crash is instructive. The plane developed a fire in one engine during takeoff, pulled off the runway, evacuation ordered, and from the time the plane stopped until there was smoke and fire in the cabin was less than a minute. Both of the rear exits were unusable, and congestion at the sole usable - and partially obstructed - overwing exit prevented passengers from using it efficiently.
Think about it. You've got only a narrow corridor to safety and less than a minute to make it. Any delay or obstruction is going to block your progress, and once toxic smoke enters the cabin, your first breath will render you incapable of further progress, even if you are doing nothing more than waiting patiently for your turn. Most of the 53 deaths were caused by smoke inhalation. Passengers collapsed, blocking the egress of those further back, who were trapped and succumbed in their turn.
The death toll would have been higher if not for the actions of the cabin crew, who were able to clear a jam in the front of the cabin. Passengers had tried to push their way along the single aisle and wedged others ahead of them into a solid mass.
The crucial overwing exit had no crew in attendance and was thus entirely in the control of the passengers. It took 45 seconds to open the hatch, and there was smoke in the cabin in less than a minute. 100 passengers had to pass through this exit in ten seconds. Unsurprisingly, most didn't make it.
This wasn't a crash. This was an airliner which aborted take-off, steered to a safe place, and evacuation ordered when the plane had stopped. Firefighters were on the scene almost immediately. But still, 53 people died.
So yeah. I'm all in favour of locking the overhead bins. The passengers should be focussed on one thing only: getting out of the cabin alive.
Modern safety standards being what they are, there's probably a bigger chance of a device catching fire in an overhead locker than of a crash or similar incident. This should be no big deal. Press the button to unlock them. If by some remote chance the fire has caused the lock to fail shut, there is a master key stored beside the fire extinguisher. It's called a crash axe.