Has a trained martial artist ever used their hair to gain advantage or attack opponents?

Your question reminded me of a Chinese Buddhist Nun, who is also a Kung Fu Master. She pulled eight (8) cars with her hair. True story! It was reported by both BBCNews and The Telegraph.

"Zhang believes her hair is instilled with special powers, giving her the ability to not only slice through paper with her plait [braid], but also support great weights." The Telegraph

There's a youtube video (in Dutch and Chinese without English translation), where you can see her pulling the cars in her last performance, before having her head shaved.

In the video, Zhang Tingting is 52 years of age. If she worked out how to pull cars with her long braid, I'm sure she figured out how to use her hair as a weapon too.

Sorry to disappoint, Grasshopper, but I couldn't find any video of Zhang Tingting practicing Kung Fu hair-slicing moves.