%ARTISTSORT% won't export to a text file. I noticed it isn't included in the fields included in the export section of the help file. Does that meant it won't export?
It is possible to use any user defined or preset tagfield placeholder name in a Mp3tag export script.
Note the different spelling of %ARTISTSORT% or %ARTIST SORT% or %ARTIST_SORT%.
Are they case sensitive? I rewrote the tage in my text.mte as lower case (even though it appears in upper case in the dropdown box in mp3tag) and it now works. Unless I had actually misspelt it without realising.