The CCG3 can be realize it by customize firmware internal. I just want to let you know CYPD3120 and CYPD3123 firmware project have already implemented USB to I2C bridge internal. The related firmware source code is bb_internal.c, usb_hid.c.
We got the same response from Lisa before and we have almost got USB HID to work now by modifying the USB Billboard device class that is included in the DP-dongle source code example. We didn't use the USB to I2C bridge which is used for firmware/debug interface, we felt that was too dangerous
We thought also about doing UART instead but that would be a larger step to go from based on the Billboard class.
We used a standard PSoC 5LP / PSoC creator to generate all descriptor file content and then inserted them into the CCG4 GW BTW, there is is just drag-n-drop to add USB HID/UART.
Lisa: You need to tell your product managers to enable the designer-view in PSoC creator to work with these block on the CCGx firmware. Then you product would be clearly better than TI's products.