Preface: I am not a developer or a command-line guy.
My company, E-Spec, has been utilizing custom embedded XMP metadata in system integrations since 2006; we have since productized our approach with Tag-It!. Tag-It! defines, collects and embeds custom XMP metadata without programing. We can simply provide Tag-It!'s custom namespace, Tag name and list of fields to a DAM system and it will ingest the XMP as it catalogs the file. We are finding many DAM vendors are using ExifTool with their systems to extract XMP. Some have implemented the tool to ignore namespaces and tags completely, just searching for field names - this works as long as unique field names are used in the custom XMP. Others only support registered namespaces so they only ingest standard XMP, not custom. For AEM, we know how to enter the custom namespace and tag in the configuration and the field names on the layouts and reports. Other DAM systems let you specify your custom Tag name and then map the custom fields to fields in their system (typically user-defined fields); they ignore the namespace completely.
After browsing this forum, it appears that you can provide a config file for ExifTool to read custom namespaces, tag names and fields. I can make Tag-It!'s web portal create such a config file and include it in our download section. Not knowing how these vendors implement ExifTool, I am not sure if the config file needs to be loaded on the client or the DAM server. Any insight on how to provide our custom XMP namespaces, Tag names and fields to a generic implementation of ExifTool?
TIA,
Dan
PS: if you are interested in learning more about Tag-It!, here is an intro video: https://vimeo.com/510769098/b1f19df178
The example.config file (https://raw.githubusercontent.com/exiftool/exiftool/master/config_files/example.config) lists where you would place the .exiftool_config file so it will be automatically read by exiftool. The easiest being the same directory that exiftool is located in.