The reason why some (but not all, maybe that will change?) CM roms have USB Audio is because of a CM dev named Steve Kondik. He's the one who has made USB Audio with (w/o UARP) on certain CM roms.
In this link, he even says he'll be able to get it work with the Nexus 5, but that hasn't been accomplished yet. Steve Kondik is a Certified Android DAC Head-Fi Hero. You can also do some google searches for him and USB audio and you'll see, he's the guy at CM working to give us that USB Audio support we crave.