fwiw; I've prototyped some USB gadgets using Raspberry Pi Zeroes and Composite USB in the Linux Kernel. At least storage and serial devices were pretty easy to get going.
You'll need something like a shell script on the Raspberry to initialize the composite kernel module and you'll find the boildplate in the kernel docs.
I would love to play with pis and make virtual usb devices, like a webcam.
pikvm is an interesting project.
It will hook to a PC, and the USB connection can not only pretend to be a keyboard and mouse, but it can be a USB drive that you can boot the system with. Pretty interesting for installs.
You'll need something like a shell script on the Raspberry to initialize the composite kernel module and you'll find the boildplate in the kernel docs.