What you would need to handle the charging would be best suited for an IC such as this one. You could make up a small charging circuit around it and feed it to a hi cap Li-Ion battery pack (probably not a notebook pack though). Also, probably not for the non-electronically inclined.

What might work better than a notebook battery would be to series some PDA Li-Ion packs. I don't know what their nominal voltage is, but assuming it's 3.6 volts, you could series 4 of them and get 14.4 volts at ~1400 mah. I don't know what the average power consumption of the Empeg is, but you should be able to get at least 3 hours out of this setup, I would think.

Stu