I doubt that headrests were intentionally made to be window smashers.  However, it might be worth a try to use one of the metal prongs for that task.  It would be unwieldy, but it might work.  A sharp impact strike in a small area will easily break out a car side window (nor the windshield though).
In all my years as a paramedic, I never saw glass broken out by a head rest.  By heads, yes, but head rests, no.