I have no idea what exactly 200.7(B) is trying to tell us, but it is my opinion that 200.6 applies to the grounded conductor of a Class 2 control circuit.
Well we can go back to 200.1 and ask if this is premises wiring, but I think it is.
The rules of art 200 go back a long way, but so does the practice of using a white conductor as a non grounded conductor in HVAC controls, so I really don't have an answer at this point.
It is also common practice to use black as negative conductor in a DC circuit, and sometimes it is grounded sometime it is not, but if it were grounded it should probably be white to comply with art 200.6.
I have not carefully read through art 725 yet to see if something is there that relieves the requirements of art 200.
OK after a quick look at 725 I did find this in 725.3: "Only those sections of Article 300 referenced in this article shall apply to Class 1, Class 2, and Class 3 circuits." Although it doesn't apply to our situation of applying art 200, I find it at least worth mentioning that other than the mentioned articles there you can forget about chapter 3 when running a 725 control circuit.