Here is some useful information for anyone intending to use a USB-powered USB DAC with an iPad, without wanting to resort to a mains-powered USB hub...
The following "unofficial CCK"/ hub will work with some DACs using only the iPad's battery for power:
http://item.taobao.com/item.htm?spm=a230r.1.10.49.tCIUHg&id=15739202629
If the iPad is not able to provide enough power (I am using an iBasso D7), you can connect a rechargeable USB booster, such as the Sanyo Eneloop KBC-E1A. This works perfectly, and provides a very clean sound. I am currently testing the battery life, and will report back!
Total outlay: RMB 34 for the hub and RMB 160 for the booster, amounting to the princely sum of RMB 194 (approx USD 31) for a universal OTG USB DAC capability....
Current OTG rig:
AIFF -> iPad2 -> hub from Taobao with Eneloop KBC-E1A -> iBasso D7 -> ZY Cable SPC mini-mini interconnect -> iBasso P4 -> HD25 with aftermarket SPC cable.
OK, so it's TRANSportable 