Not sure that cases might be a theoretical no. Even if it's a device that Android isn't aware of, an Android application can access it directly using USBManager.
From a Moto Mod standpoint, you don't have to worry about all the power and signalling for OTG. In the USB-Ext protocol, you define your path and protocol, as well as whether you're a host (USB3.0 only) or a device.
