Can I safely substitute a DC converter for the Macbook's AC adapter?
If you have to do this, I would not mess on the Magsafe adapter site.

Instead get one of the commercially available Car charges and there you can slice open the car plug if you have to.

Here is a example of such product 

Doing it your way, you will still need a DC-AC-DC step up from 12 to 14.6 Volt. Furthermore you would have to be very careful with the Magsafe plug and the super small space to work with, and the risks involved.