I guess I'm the only one interested in connecting my phone to an external DAC. I ordered the N5 anyway and will test it out as soon as I receive it. I'll post back my results as soon as that happens.
I just saw this thread and I care deeply about this. First thing I did when I turned on my Nexus 5 today (after installing the required update, of course) was try my USB DACs, the hifimediy sabre and Fiio E07k.
Neither worked. When you plug them in, and have your headphones connected to the DAC, you hear a click. But then you go to play music, and the music comes out of the phone's speakers. I even tried putting the phone into debugging mode. No dice.
Did not try the USB Audio Recorder Pro App because I have no desire to use that clunky app, I simply want to easily use my USB DAC via OTG with my new Nexus 5 and after that leak a month ago from the Android 4.4 dump, my hopes got up. They have been dashed now.