Very interested in finding a solution too. I think the below item would work. The headphones only/won't data sync are what most of them are. It's not nearly as expensive as the irig powerbridge ($80). Seems like a simple hack to route the 5 volt from a different source though....
If this does work I would expect that you would get noise unless you use a battery powered usb charger.
https://www.compuclevergear.com/products/iphone-7-lightning....
Here's a cheaper version on Amazon: http://a.co/d/fwvhCGu