Is there any way to create visible annotations to a .tig or .jpg using exiftool?
By this I mean an insert of perhaps a visible index number that might appear in the lower corner of the image, as in the .jpg, attached?
No, exiftool cannot edit actual image data, only the embedded metadata.
You might look into ImageMagick (http://imagemagick.org/) if you need to edit images on the command line.
I use imagemagick to write a copyright on the versions of my photos that are on my website.
It is supposed to be very clever as the same copyright stamp can be used against both dark and light backgrounds. It doesn't do very well against mottled though.
http://clifford.ac/photo/2021/Jan-Feb-Mar/tn/p340_1985.jpg.index.html
http://clifford.ac/photo/2021/Jan-Feb-Mar/tn/ahc_8314.jpg.index.html