Editing the contents of a single column at a time

[ohrenkino]

@Wolksby

contents in, say, a notepad file

If you want to edit the list in an external editor, you would have to use the import and export functions like you did for Genre.
As has been said in this thread:

Customising tags for my specific MP3 collection

In short: Yes. Except for _SEPARATOR all field definitions for the tag panel behave in the same way. A look at the documentation would also have revealed the features:

All the tag panel fields behave in the same way in respect to custom list entries.