The 'Automatic SPDIF (Optical/RCA) Audio Switch' from Tindie (link in first post) came quicker than expected. I ordered the version with 4 optical / 3 coaxial inputs and 1 optical/coaxial output.
This unit has one single 'issue' for typical RME users: it doesn't speak ADAT, because it uses a special SPDIF transceiver inside.
Apart from that I am really blown away. This small unit does everything right, works perfectly, and passed all my tests without any problem. It works as true pass-through (24 bit audio data and channel status), supports all main sample rates from 32 kHz up to 192 kHz, and selects the inputs automatically as logical (and useful) as possible.
The one thing that astonished me most: the internal chip, a WM8805, offers jitter reduction in the audio band. The Wolfson data sheet does not talk about that at all. Yes, this thing reduces jitter by at least a factor of 40 where it matters, without the use of a SRC. A very rare feature, even more so for such a device.
SPDIF coaxial starts to detect an input signal at 0.3 Vss, but after locking works down to 0.15 Vss, so is very reliable and compatible. The coax output is transformer-coupled and provides 0.6 V (standard is 0.5 V), just the little bit of reserve that one wants to see and needs in real-life operation. The optical I/Os use DLR/DLT2160, the right ones for successful 192 kHz operation.
All I can say is: Highly recommended! Now the only thing missing is another version with remote control...
Update: Here is the jitter suppression measurement. Works even better/stronger than I first wrote. Int Loopback is the AP's Jitter generator loopback to its digital input. At 200 ns jitter (2.5 kHz modulation), the Auto SPDIF Switcher removes the jitter nearly completely. The measured jitter damping is higher than 50 dB.