Can I use a pin/password/pattern lock and safely have basic phone functionality while in car?
There are programs like Tasker, and llama that can automate things like that for you. But the problem here is how to tell those program that you are inside your car. If the conditions were like time of day, location or battery percentage you could easily use those programs to accomplish such task.

In programs like Tasker and Llama, you can create task so that your lock screen will be disabled when you plug in headphone. But the problem here is, if anyone, anywhere plugs in the headphone your lock screen will get disabled.

Also Sony recently release NFC tag.

http://www.psfk.com/2012/01/sony-nfc-tags.html">

Maybe you can have one such device in your car and provided that you have NFC in your phone you can just tag it and the lock screen will be disabled. And maybe tag it again before leaving the car to enable the lock screen again.