I saw a Radio Shack wall charger that lets you select the volts, has a USB connection option, and puts out good enough amps. Plug a plain 5V passthrough into it and select the voltage you want at the wall charger. That would be the only way I can think of to get there.
A vv passthrough
mod would have to take a higher input voltage, 7V or higher, and then regulate it down. So because USB supplies only 5V, you can't make a USB passthrough that is vv.