Hey there.
One of my ideas/concepts at the moment involves car access via NFC ring.
What I'll be looking at doing once I have a ring to play with and have finished a couple other projects is to see if I can use an arduino with an NFC reader to trigger a relay that operates an electric door latch of the type used on hot rods etcetera when the door handles have been removed.
The triggered latch then moves freely and allows the door to be 'popped' by a spring fitted to the frame.
The concept should be achievable, though I haven't even started to look at it - issues I can see straight out are the receiver loop and where it will be placed. I'm unsure what the effect will be of a large amount of metal in the surrounding area. (I have a 1970s american car.)
This might be easier with a newer model vehicle where the door handle area is largely plastic.
With this setup there would be no real need for a 'lock' action because closing the door removes the possibility of re-opening it without having the ring or knowing the 'emergency' procedure. The only need for the ring then is unlocking and popping the door, which should be suitably dramatic - reach towards where the door handle used to be and the door springs open.
There are plenty of hot rod websites that sell these door kits, and there are heaps of choices for your NFC reader in far more sane packages than a smartphone strapped to the car.