Obviously if the hardware is all integrated together, then you can't do isolation as I described. However, in your parent comment you used the example of a GPU

Obviously if the hardware is all integrated together, then you can't do isolation as I described. However, in your parent comment you used the example of a GPU, which is something that can perfectly be fined be isolated, and provided you can write an open-source driver and can bitsniff all the data coming in and out of your untrusted GPU, then you can be reasonably sure it is not for instance sending your screen data to your network controller.