That'd be only slightly more complicated. Based on the need for literally ANY smartphone to work with this I'd imagine it'd have to be through a group like onstar that's "always connected" to your car. And likely run it through bluetooth, unless we wanna chance it running a wireless AP through the car. But that'd lead to timing concerns(how fast the two pieces establish a secure connection and finish the "handshake" process). And require the car and phone both update with a new random encryption key every 24-48 hours for security reasons. You COULD run it through the same process as a vpn key, or game authenticator. But in that case your code is out there in the world for people see/access.
That whole thing is very much already possible. Just needs someone to code for it, and a large group(like onstar) to be willing to take the risk.