Hi Big_Berny, just being a new user of Mp3tag you have to learn how to apply the general functionalities and the specialities of Mp3tag.
Mp3tag offers a bunch of tools to make tagging life easier.
If it goes to special user wishes, which are easy to solve within other open scripting languages, then often it leads to rather complicated think-around-the corner constructions.
Beside some ready to use "Convert" and "Action" dialogs, Mp3tag has it's own scripting language to solve specific user defined tasks. You know VBScript and the learning curve will be not so steap.
Although Mp3tag does not support any white or black lists as you need it, you can use Mp3tag scripting language to make your own lookup function.
If you need a "Variable", then create a user defined tag field with a name of your will, e. g. by using an action "Format value", and fill the tag field with content as you will.
If you want to destroy the variable, then use an action "Remove tag fields".
There is also an action "Import from textfile", which can import a complete text file into a tag field.
Back to your current problem ... you need a lookup table with two columns.
Column1 has the search item (the key), column2 has the target value (the value).
There are examples how you can solve your problem ...
/t/4458/1
/t/5660/1
Good luck!
DD.20100417.0736.CEST