There is really no such a thing called 'USB digital out' - at least not in the same sense as SPDIF type of digital out for the audio world. Standard like SPDIF sends pre-defined digitized audio signal (well, audio date stream) to an external decoder (i.e. DAC) to convert back to analog signal. USB however doesn't send audio specific data. It sends digital data and command in serial - any kind of digital data is fine, then there is another I/O chip on the other ends will figure out what to do with the data and command. These data can be stored into a SD card or it can be sent to a DAC depends on what command is sent.
So the problem is that USB is a much more complicated system compared to just audio data, and therefore you need to implement both the source as well as the receiver. When the two connected, the source need to tell the receiver what to do and set everything into the right path. In SPDIF, everything has been pre-defined so both ends already know what to do and need not communicate as much. That is why to use USB DAC, you need a much more complicated system, and often involved (but not always) an OS.
Can it be done on X3? Yes.
...But on the current budget without any other compromises? No.