The downloadable Exif Fixer app for macOS, Windows and Linux (NOT the Online web version) can be used to add any additional kind of exif metadata to your images as long as you know how to write correctly-formatted data.
I will add more custom exif metadata tips here as I find ones that seem useful.
Some platforms can make use of geotagged metadata to show the photo’s location in a map. To add GPS coordinates for the photo location to an image type the following
-XMP-exif:GPSLatitude#= (followed by the latitude coordinates)
-XMP-exif:GPSLongitude#= (followed by the longitude coordinates)
You can use the map at https://www.gps-coordinates.net to find the correct latitude and longitude coordinates for anywhere on the planet. Use those with the above lines (remembering the dash character at the start of each one) in Exif Fixer’s metadata list.
Exif Fixer Online is free to use, just like the standalone Exif Fixer app, but any tips help offset the cost of my software tools, time and hosting and really are gratefully received. You don't need a PayPal account to use this donation button.
Many thanks to everyone who has tipped me! It has directly helped support the development of this app and service and I really appreciate it.