Thanks @defragster and @tni!

I am looking at trying to handle the Teensy configured as an USB mouse as a mouse to the T3.6...

The first issue, is the USBHost code is not currently claiming the Mouse. This is because it is not marked as a Boot device nor as mouse.

From ...cores\teensy3\usb_desc.c
Code:
#ifdef MOUSE_INTERFACE
        // interface descriptor, USB spec 9.6.5, page 267-269, Table 9-12
        9,                                      // bLength
        4,                                      // bDescriptorType
        MOUSE_INTERFACE,                        // bInterfaceNumber
        0,                                      // bAlternateSetting
        1,                                      // bNumEndpoints
        0x03,                                   // bInterfaceClass (0x03 = HID)
        0x00,                                   // bInterfaceSubClass (0x01 = Boot)
        0x00,                                   // bInterfaceProtocol (0x02 = Mouse)
        0,                                      // iInterface
        // HID interface descriptor, HID 1.11 spec, section 6.2.1
        9,                                      // bLength
        0x21,                                   // bDescriptorType
        0x11, 0x01,                             // bcdHID
        0,                                      // bCountryCode
        1,                                      // bNumDescriptors
        0x22,                                   // bDescriptorType
        LSB(sizeof(mouse_report_desc)),         // wDescriptorLength
        MSB(sizeof(mouse_report_desc)),
        // endpoint descriptor, USB spec 9.6.6, page 269-271, Table 9-13
        7,                                      // bLength
        5,                                      // bDescriptorType
        MOUSE_ENDPOINT | 0x80,                  // bEndpointAddress
        0x03,                                   // bmAttributes (0x03=intr)
        MOUSE_SIZE, 0,                          // wMaxPacketSize
        MOUSE_INTERVAL,                         // bInterval
#endif // MOUSE_INTERFACE
Again as you can see it is not setup as boot device...

Currently the USBHost mouse claim code checks these values:
Code:
	if (descriptors[5] != 3) return false; // bInterfaceClass, 3 = HID
	if (descriptors[6] != 1) return false; // bInterfaceSubClass, 1 = Boot Device
	if (descriptors[7] != 2) return false; // bInterfaceProtocol, 2 = Mouse
And bails from trying to claim it... So the mouse does not work...

I obviously may know enough here... But how does Windows know that this is a mouse? As I can plug it in to my PC and with the Triangle example sketch, my mouse moves itself around in a triangle pattern.

Suggestions?