IQAudio sure know their stuff. Really nice folks, too!
Some guys on these forums have come up with solutions for powering display/Pi from the same, more powerful, power supply, and I definitely understand your desire to retain a plug!
A small, powered USB hub with a 3Aish power supply should do the trick. We should probably test some options for this and come up with a known good solution, but right now there's just no time. Negative time, in fact!