I think you want to look at the message parameter
http://svnbook.red-bean.com/en/1.0/re07.html
svn copy .... --message (-m) TEXT
EDIT:
Nothing. This is not what you want. You want to look at the torso to see which copies / tags you made ... You can do this:
svn list --verbose e.g. svn list --verbose file: /// repository_name / tags
But in the end, I think you answered your question. Store the text file somewhere and during assembly add it automatically with the tag name and then check it with the tag / tag comment.
I did something similar in the previous company. The build script had its own user input in SVN and was checked in the version file.
i.e. /Project/Builds/builds.txt
. , . , , . CI/many builds .
, .
, , tag-revision-n +1 .