I´m surprised nobody has mentioned the late Nicolette Larsen. Absolutely amazing, thick, classic-length, wavy hair, beautiful face, very natural aura. For me the most stunning woman I´ve ever seen (unfortunately never in person).

Or am I defining "rock" too loosely?

If you consider singers in general, there seems to have been an opera singer named Blanche Thebault or Thibaud (please double-check on spelling) who in the 50s and 60s grew her fantastic brunette mane to the floor.

Btw, the longest hair I have seen in real life was a woman in her early 20s probably (back in 1988), at an open air Brazilian music festival in Cologne, Germany. She had healthy and shiny brunette hair in a braid just past her knees, and the rest of its length was flowing down freely (without thinning at all) to her ankles, just touching the top of her feet. She didn´t show the faintest hint of Asian or Latin descendance, looking just like an average (but beautiful) European.