Ok, so I ordered a lightning to USB-B cable from Amazon and received it today, and lo and behold, it worked! I'm using my son's ipad mini 3 running ios 12.1.4. I plugged the cable into the ipad, then into the DTX502, then turned on the module. The app displayed a message that the firmware on the 502 needed updating. The update process was simple and took all of three minutes. Upon initial use of the app, it seems very straightforward and easy to adjust settings. The app window is iphone screen-sized, so you have to press the expand icon in the bottom right corner of the ipad to make the app larger.
To conclude, it seems that the usb camera adapter is not necessary to connect the 502 to an ipad with a lightning connector; just a lightning-to-USB-B cable. I also downloaded the Yamaha Visual Performer app, which my son likes to watch while he's playing. Overall a great success!