I note that in the AIVDM/AIVDO protocol decoding document, you have some preliminary notes regarding NMEA TAG Blocks.
I have several million examples of real-world tag block usage and a pretty good handle on the tag block meaning.
Would you like me to send the info and a sample of actual messages with tag blocks to you?
In my opinion, If tag blocks were implemented the tag block data should become additional optional fields within the "class": "AIS" JSON output by gpsd / gpsdecode.
I'm uncertain whether the data and information that I have reflects IEC 62320 or NMEA 4.
p.s. The tag blocks are the 'metadata' that I have referred to in the past and would, if implemented in gpsd / gpsdecode, replace my need for JSON passthrough. I just didn't realize until recently that this 'metadata' was actually called 'tag blocks :-)