I know virtually nothing about iPhones, but, my Android device can act as a USB host, so with an OTG connector, I (theoretically) can connect to the DP machine. I'm going to try that and see what happens. It would be interesting to see if the Lightening-to-USB connector works with the DP machine on the iPhone.
But, a card reader will definitely work because I've used those, USB thumb drives and USB Hard drives with my phone and an OTG connector. I was able to read and transfer files on all of them.