I remember reading about the discovery of minoxidil’s hair-prowess, and IIRC it could increase hair production anywhere.
In the clinical tests, runoff promoted hair growth in less desireable places, like on the ears and the back of the neck. Now this may be because there are already hairs there, and the minox turned the peach fuzz to wild hairs.
I’m guessing from my limited knowledge of the regulatory process that since the product was tested with baldness in mind, it was never tested (on a full clinical basis) on other parts of the body. So, a giant CYA in case someone decides he wants a thicker plume of pubic hair and discovers that the alcohol base causes dryness and itching on his scrotum.
Common sense would lead you to believe that if it’s safe for the head, it shoud be safe elsewhere. But as always, try a small inconspicuous spot first!