Rockbox only uses the Line Out and Headphone Out of the iPod right now. At the moment we're currently unable to control the USB port.
In the future, if we ever have USB drivers, it MAY become possible to do as you describe. It depends to an extent on the manner in which we can control the USB port. On some devices, such as the H100, where the USB port can only be slave, we cannot do such a thing. But on devices where it can be host, it may be possible, and it is theorized (though I don't believe there's any hard evidence yet) that the iPods may fall into this category.