Tie lighting on Knob and Tube to a low voltage (12v) system?
The NEC, except for a few exceptions, doesn't really concern itself with anything under 50V. There used to be some commercial showroom lighting that actually did what you are talking about back in the 90's, but I never saw any of it applied to residential. 

I would make sure of a few things before I did it.
1. Make sure the transition from 120V to 12V was code compliant.
2. Make sure that whatever you are installing is an approved system for your use.
3. I would still talk to your AHJ and get their input before doing anything.

Hope this helps and good luck.