Electronic – Dual Role USB device on AVR

Having looked at this, I think it's worth looking at Wireless USB "wire adapters": http://www.usb.org/developers/wusb/docs/presentations/2006/Taipei06_RI_Wire_Adapter_Model.pdf (lots of detail)

However, I don't think those are transparent. If you can't find a transparent one, and it's an absolute requirement, I think you'll have to settle for proxying. Plug in a device on one side, have a "host" read its descriptor and pass it over the wireless, have the proxy present that descriptor on the other side. Store and forward requests; do link-level acknowledgement on the proxy. This should work for HID devices and you can probably make it work for mass storage devices. You will probably have to do special interpretation of some types of message, so build that into your software from the start. This is basically Anindo's solution. I'd estimate it as a good few weeks of software development; once you have the basic XBee devices working you might get better answers on the software stackexchange.

Normal USB hubs are non-buffering and have a very small latency limit of a few 12MHz ticks.

If you don't mind sourcing and using a dedicated IC for the task and are willing to sacrifice a bit of board space, you can put a LTC4417 to work at this job, along the lines of the circuit below. You'll have to tweak the values to fit, though: I did a "rough cut" at the datasheet procedure for applying this chip, but you'll want to go through the datasheet application procedure yourself to make sure you're getting things like inrush currents and voltage rail droop right.

^{simulate this circuit – Schematic created using CircuitLab}

The good news with this solution is that you can't do something silly with it inadvertently, like backpower the USB port, and it takes care of all that prioritization business for you (the IC always prioritizes V1 over V2 when both are functioning).