I've had a couple times recently where I've been curious whether Android usb-gadget support is exposed to devs at all.

Appears to require their other tool, which relies on root. https://github.com/tejado/android-usb-gadget

Huge props to the dev. Looks very well done. Huge passion here. Great initiative. Alas that stock Android is so hampered, offers so little to consumers. For their safety of course!