The easiest method I've found to interface an iPad with a generic USB peripheral (not listed in HT209186) is via a Raspberry Pi. I plug the USB device into the Pi, run a server that talks to the appropriate USB driver and serves the data from the peripheral using a network connection to the iOS device. (I currently have an app that uses just that method to connect to USB RTL-SDR radios.)
A wired connection for higher-bandwidth or security is possible via:
iPad Pro -> USB-C-to-Ethernet-dongle -> Pi -> USB peripheral.
Or:
iPad Mini -> Lightning-Camera-Connection-kit -> powered-USB-hub-> USB-to-Ethernet-dongle -> PI -> USB thingy.