Some more experimentation:
I disabled CBBE, Bodyslide, and Sporty Sweat (just a specular map), then started a new game. I picked the Breton female character and played to the point in Helgen Keep when your hands are cut free.
Then I cheated myself an ebony helmet and tried to equip it. No dice, hair just vanished.
I pulled up the racemenu and cycled through the races, and the helmet rendered just fine on every damned one of them except the Breton female.
Wondering if it might be some sort of cached data issue (since my main character is a Breton female), I started another new game and chose the default Nord female and repeated the whole process. This time the helmet rendered fine on ALL races, including Breton.
Curious, I tried with a Guard Helmet, since that one causes only the top of the hair to vanish, leaving the longer fringes in place. The Guard Helmet does not render for any of the humanoid female races, but it renders fine for the Argonian & Khajiit females, and for ALL of the male races.
I then loaded up my current game character in Breezehome, used racemenu to change her to the default Nord female with an ebony helmet (rendered fine), then walked her outside. I called up the racemenu again and switched her back to a Breton, albeit the default one. The helmet rendered just fine. I called up racemenu again and made a few changes to the nose/chin/hair using the sliders, and when I tried to equip the helmet, it was back to Mr. Clean.
I repeated the process, only this time I made the slider changes to my character while the ebony helmet was equipped, and it seemed to work fine.
So I have no idea what's happening.