I finally sat down today and wrote the firmware for that first module. It's rather simple, the joystick does mouse movement, with some dead zone in the middle (I can still improve the dead zone calculation, but it's good enough), the knob does scrolling (and I should add the ability to change volume using it with some function key), and the buttons are mouse buttons.

    def send_mouse_report(self):
        report = bytearray(4)
        report[0] = ((not self.lmb.value) << 0) | ((not self.rmb.value) << 1)
        knob = self.knob.position
        if self.last_knob != knob:
            report[3] = max(-127, min(127, knob - self.last_knob)) & 0xff
        self.last_knob = knob

        x = self.mx.value - 0x7fff + 200
        y = 0x7fff - self.my.value + 500
        if x * x + y * y > 4000 * 4000:
            if x > 0:
                x = max(0, x - 4000)
            else:
                x = min(0, x + 4000)
            if y > 0:
                y = max(0, y - 4000)
            else:
                y = min(0, y + 4000)
            report[1] = min(max(-127, x >> 10), 127) & 0xff
            report[2] = min(max(-127, y >> 10), 127) & 0xff
        else:
            report[2] = 0
            report[1] = 0
        if report[0] or report[1] or report[2] or report[3]:
            self.mouse_move = False
            self.mouse_device.send_report(report)
        elif not self.mouse_move:
            self.mouse_move = True
            self.mouse_device.send_report(report)

Not much to say, really.