You could send a message to Khadas staff like Gouwa, tsangyoujun and the team.
I do recall optical SPDIF, as well as balanced outputs, being discussed for a future revision. I am on an old phone now, searching for links is laborious on it. When I get to a PC, I will find that discussion, will be later.
The same signal drives both coaxial and optical SPDIF, I believe it would be possible to DIY add or convert the TB’s coax(RCA) to SPDIF, but don’t quote me on that.
I believe it is possible because of an SPDIF project I did on the VIMs.
Update:
Link to balanced output discussion.
Link to SPDIF discussion.