i made a cable, that has 2 USB-A at the computer end and the other end is a USB-B + firewire.
the cable has got a boost that converts USB to 12v.
i have successfully charged 2 empty BP915 batts with this method.
the boost circuitry is a little on the big side. 4cmx1.5cm
I wonder how many people are actually interested in such a cable.
I haven't tested it on a MBP or any mac.