Please have a look at this example I threw together. It simply enumerates as a HID gamepad, then cycles through pressing each button and joystick direction on a timer. See readme for how to view the gamepad on the host OS.
It is derived from the current 19.1.2 release. I fashioned it like other programs in testhal/STM32/STM32F4xx/. If acceptable to you, please feel free to include it with future releases to help others (and future me). I relinquish all ownership/copyright/claims whatever to ChibiOS (most of the code was just copy/pasted from other demos anyhow
You'll need adjust the CHIBIOS directive in the Makefile to wherever you place it in the tree. Also please double check that the CH_DBG_XYZ and compiler options fit in.
Best,
Dave