There is a CyanogenMod 10.1 based rom that combines Timur's kernel with the jacknorris mods. It supports hotplugging, usb charging, FI etc. Everything is configurable in the settings menu. It's pretty slick. http://mehrvarz.github.com/nexus-7-usbrom/

I'm having a similar issue with my Ayre QB-9. It works with USB Audio Recorder Pro with the default settings, but there is no sound with any of the Nexus 7 kernels I've tried (Trinity, Faux, Timur, jacknorris). While the QB-9isn't a portable headphone dac, it does use the Streamlength async usb...

Timur's kernel supports battery charging with a usb otg Y cable: http://rootzwiki.com/topic/37755-timurs-kernel-usb-host-power-management-usb-audio/