DC power from an airline headphone jack

This company has an interesting device that "Simply by plugging in the cable to the passenger seat headset jack, the cable converts the audio output into regulated USB power."
Inflight Power

The Inflight Power cable uses an internal trickle charger that takes 3-5 min to fully charge, then outputs power to the attached device (phone, music player etc). It does this repeatedly, thus the cable charges the attached device every 3-5 minutes for 1 minute or less.

The company also offers standard "Gomatic" tips that convert a USB connector to a device-specific connector for DC power.