If you mean plugging a USB OTG cable onto the X3 and connecting a USB flash drive on it, then I just tried it and doesn't work. X3 doesn't see anything being plugged in.
If you mean plugging a USB OTG cable onto a smartphone and connecting the X3 onto it, then it works. I was able to use the X3 as a DAC from my Xperia without needing additional bus power.
The former, sadly.
Does anyone have an Apple device running 7.x laying around? There was an user who couldn't get it to work in this setup. It would be great to stream hi-res files to the x3 through Plex or even iTunes Sharing.
Maybe the latter is what James said a while ago. I'll dig up his post later.