If we are talking about Bluetooth, you are not going to find any wireless keyboard with USB ports. There are technical issues besides the battery power on this,
1) The keyboard must have USB-to-BT bridge to convert USB signal to BT, which I don't know the existence of such chip,
2) On the Mac main unit, it then must have BT-to-USB bridge, which it does not exist.
2) BT speed is about 721 Kbps, USB 1.1 speed is 12 Mbps. No matter whether it is 721 Kbps or 12 Mbps, it is too slow for an external HD. (FYI, Apple wired keyboard only support USB 1.1, not 2.0).
While Wireless USB (Intel) or Firewire (1394 Consortium) spec is ready; however, there is no such product out there yet.
I don't know there is any non-BT wireless keyboard with USB ports out in the market. But even if there is, you need to attach the base station to a USB port. And more likely, the USB port is only 12 Mbps (ver. 1.1)