I have done this in the past using H-E-N with vbus boost on to give me an extra hour or two of music time on my BH-214 headset before its battery upgrade, but more importantly I have used it to emergency charge my wifes mobile phone while we were on the train, 15min gave her about three hours standby and one bar.
Just remember that the microUSB port is fragile, I wouldn't make this a common option but for emergencies as leaving a USB port plugged in to your phone puts it at serious risk.