USB 1.0 can supply 500ma down the USB bus lines [part of the spec] and i'm sure USB2.0 would supply at least that, if not more.

500ma while not a lot and shared across multiple devices would allow at least a reasonable charge rate while still running the device - especially if like the empeg the disks are only running when the unit is playing music.
And if you leave it docked all day, it should be able to charge it up enough to run it during the [long] trip home on the bus or train or whatever.

If you need more current a external USB hub will be able to supply such power - I have a Belkin USB (1.0) hub thing at home that comes with a wall wart that enables it to supply 500ma down each of the 4 USB ports it has on it.

If you plug your emPod* to your USB bus on a dedicated port, then I would think the system can supply it with enough juice for it to run and charge the batteries even if a trickle charge.
Over 8 hours that would be enough to keep it topped up I think.






*emPod name and associated logos are Trade Marks of Everyman Enterprises :-)))