But the point is that I want open source drivers, and open source hardware! I don't want to depend on the health and the priorities of a commercial entity. I want to be able to inspect the driver software and contribute towards perfecting it!
Compared to, say, GPUs, the needs of multichannel digital audio i/o are modest and not changing very much over time.
Some quick usage of Wikipedia suggest that you are, sadly, mistaken. ADAT lightpipe seems to use a completely different protocol (more capable?), and while S/PDIF and AES/EBU use quite similar protocols, they differ in impedance and max and min voltages, so I'm guessing that directly electrically connecting the two controllers is a bad idea for both the controller and the data.
(Licensing could be a concern though if ADAT lightpipe is proprietary — I'm not clear on that. But then we could just use other open protocols instead.)
If I'm not mistaken the underlying transport protocol is the same for all of those - you can get a coaxial to XLR to get S/PDIF into an AES port or a coaxial to TOSLink converter to get S/PDIF data into an ADAT port.
You can look into the Madiface XT maybe ? https://www.rme-audio.de/hdspe-madi-fx.html or the other RME products which have enough digital I/O to cover a lot of needs