Its a little difficult to understand how the rules can refer to "The peak to peak variation in visual signal level caused by undesired low frequency disturbances (hum or repetitive transients) generated within the system, or by inadequate low frequency response" and allow measurements to be performed on a single unmodulated carrier. How does running the test on an unmodulated carrier check for inadequate low frequency response?
Regardless, you're only required to conduct this test on a single channel or unmodulated carrier. In practice, its a good idea to run it on at least a couple of channels -- one at the low frequency end of the spectrum and another at the high frequency end.
This test can be done with the channel in service. Therefore, its a good idea to "run through" all your channels to catch things like hum caused by bad power supplies.