Implementing I2S over HDMI (via LVDS) is pretty straight forward. Not sure about the USB though. The limitations with the pi regarding USB are well known. Adding a USB port via a USB hub would do nothing for the performance.
I have been giving some thought to modifying our 502DAC and adding I2S where we have the status LED’s. I will post more about this on the SBAF forum soon.
http://www.superbestaudiofriends.org/index.php?threads/raspberry-pi-i2s-to-spdif-hat.1990
I do not spend much time here since we never did create a RAAT for our 503HTA, and will likely not have the resources to do it for our new DAC boards.
Cheers,
Michael