Sorry I didn't see your edit as I was writing my response.
If the hardware I2C port is available then that is the best option, far less impact on any existing timing code that you have as well compared to bitbanging. I've used these displays with I2C at 1Mbit/sec in the past then they performed fine, even when doing high frame-rate animation.
Fully appreciate the queue of existing work you have, it was why I first asked about any plans to open-source the firmware. At least now I know that suitable pins are already available on the current hardware, so it's at least possible.