And this does work.
I use a set of Sony bluetooth headphones with a built in mic and rudementary controls that control the phone (volume, fwd, back, enter buttons)
I was running the other day and my wife texted me a question. After hearing my message received tone, I hit my headphone return button and told Siri to read me the message. It asked what I wanted to do and I said reply and stated my response. When it asked whether to send or not, I made sure it accurately received the message so I had it repeat it back to me and then send it.
The thing that may trip it up is the location of your mic on the headphones. Because mine was on the headphone itself I had to say it pretty loudly. Those that have mics on cords or closer to your mouth will probably have even better results than me.
After my first text response while running, I just thought technology is starting to sprint forward pretty fast.