I've got a 13-port USB hub which comes with a 5V 4A PSU - more than enough current to power the bifferboard, the hub, and any devices plugged into the hub (I haven't got a use for all 13 ports yet!)
So I bought some connectors (one female, two male) and soldered up a simple power spliter cable, as shown in the picture below. If you try this yourself, check that your hub takes the same 5 volts as the bifferboard, and double-check you get the polarity of each connector the right way round!
I got the tiny USB A-B adaptor as part of a cable travel pack
bifferboard can be powered directly by the hub. I did some tests, and out of the 6 different self-powered USB hubs I have, only one (pictured below) was able to "reverse power" my bifferboard. So I guess you need to try it and see!
From Biff: This also works like that: