Long answer with technical explanations: Let's clear up a few things first. The uDAC means u (ancient Greek 'mwu', I think it kinda says/suggests 'micro' in English) Digital to Analog Converter. That means it gets 1's and 0's of digitally coded audio then translates that to analog electronic signal, what you see as wave measurements. In the case of the uDAC, it
1) takes digital audio via USB - no SPDIF or other digital protocol; its power supply design does not use a transformer or battery and charger, but uses a 'charge-pump- circuit taking 5volts off the USB source, as USB was designed for (this is a lot like a charger circuit without a battery on the uDAC)
2) outputs the analog signal to either a transducer through the RCA sockets, in this case a headphone through the front socket, or an amplifier that powers a transducer that requires more juice, such as a dedicated headphone amp into a power-/current-/voltage-hungry (in other words, high impedance and/or low efficiency) headphone, or a speaker amplifier, or an active speaker (a speaker with the amp built into the cabinet; eg, computer speakers and most studio monitors)
Which means that if you connect a USB charger to the USB in of the uDAC you'll feed it 5v to power it, but you have no digital signal - or any sort of signal - going in. And connecting an iPod or any source through the RCA is like driving the wrong way through a one way street, and since it's connected to a USB charger without any input, it basically means you won't run into anything head-on.
So the short answer is: no it won't work.
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As for the ProJect Headbox, if it has both USB digital and analog RCA inputs, then yes you can use it to bypass the (crappy) soundcard on the PC, and then use it with an iPod using the lineout dock to the appropriate cable termination (RCA or 3.5mm stereo).
