Whilst I was doing research in HDMI -> MIPI, I wanted to see if there was a way to get USB-C Alternative Mode video out from HDMI too, since the CB1 has 2 HDMI ports. It wasn't long before I went through a hyperlink into this PDF about the ANX7688:

Cool. That sounds great. Is it on AliExpress?

Noh... It couldn't be... Is there another chip with the same name?

Unreal engine!

Anyway, it wasn't long before I found https://xnux.eu/devices/feature/anx7688.html, where it sounded like there was some linux drivers and firmware written for it. It's featured in a device called the PinePhone, and it turns out that there's a schematic for it! The chip in question is on page 13, so there could be a chance that I don't even need to read a datasheet to implement it! Truly exciting.