Using this batch script operation as an example:
rem Saved in D:\tag data\bat\run.bat
@echo off
@echo f37c537b-3557-4031-bfd6-ab63ced32854>> 104394872.txt
@echo 4f1949ab-fefd-4bf3-9317-814084c29cc0>> 2619264081.txt
@echo f9af11e9-40c4-42c0-8ea9-99b9c84868057>> 3074071749.txt
The long set of numbers are musicbrainze_artistid and the second set are unique key identifier for an artist to which the mbid_artistid corresponds. You could use artist name instead of a unique key identifier but bat operations isn't unicode so this tends to give problems with special characters. A set of numbers are no problem however and I had those already since I use those for data import into an sqlite database.
First I search for all metadata that has musicbrainze_artistid present, then I use text tools -> advanced and fill in my track pattern also including the commands for the bat script: @echo %FORMED/BORN%>> %FOO_ARTIST_ID%.txt. I select skip duplicate/repeating lines.
Then I copy paste the entire selection in excel to further delete duplicates since text tools doesn't fully do that.
Then I copy paste from excel into a txt file and add the top two lines to the batch srcipt
rem Saved in D:\tag data\bat\run.bat
@echo off
(path must be adjusted to what you use of course)
Then I run the script and it will result in multiple text files with each text files named as the unique key identifier with each file having the musicbrainz_artistid corresponding to that identifier (and subsequently to a specific artist).
The rest you know, I use your method via MP3Tag top populate my Musicbrainz_artistIds. The benefit here is that I don't need to add a Musicbrainz_artistIds. to newly added files when it is already present for that artist in the library.
Btw, if you want to use a key identifier for your artists like I use, this is the formatting pattern using masstagger (format value):
$if2(%FOO_ARTIST_ID%,$crc32($lower([%artist%])))