That's cool, and I thought about doing getting of of those. Last festival I took the poor man's equivalent... a spare car battery and an inverter, and tried to do the same thing, but it didn't work nearly as well as I had hoped (maybe my old car battery is junk). I think maybe Inverting from 12VDC to 110VAC, then using a 110V > DC wallwart to charge batteries is inherently inefficient.
Alternately, you could take a cord from the cigarette lighter plug on the B&D and run it into the DVD player batteries (one of my wall warts puts out 12V so they will take that). I suspect you can charge many more batteries this way than you can by DC > AC, and AC > DC because it skips that extra step. I haven't done this yet, so I don't have any real data to back it up, but I think it makes sense from an EE standpoint.
But kudos for trying this, and if it works great I may do the same thing.