What is the problem with using a bayonet-to-thread adapter?
Concerning a custom made adapter, I assume SK Grimes in the US are the right firm. But basically any local macchin shop, specialized in fine turning and milling, can do such.
Concerning that SCA adapter:
The SCA system was only applied by german flash manufacturers, who made no SDRs themselves. Their intend was to keep foot in the flash market when camera manufacturers started to make own flashes with proprietary functions (dedicated and TTL flashes).
So there is no "SCA-compatible camera" but rather a SCA adapter for some camera.
"I imagine that it should be possible to translate the signals in the opposite direction."
The SCA-system already yields a two-way communication.
What you have in mind is building a new interface with information derived from a Hasselblad and a Nikon SCA-adapter... I widsh you luck..
There are much more simple technical solutions.
The most simple solution though is to buy a SCA-ringflash and the Hasselblad SCA-adapter. Such exists.