Thanks for your help.
I understand the next loops, but I see that what I want to do is not possible in MP3Tag, and will be quite straightforward in excel, so that's the route I'll take. Maybe you have seen my other post explaining how I will do it (and with another question about the txt>tag conversion)
I didn't know that the TAB character can be used both by excel an MP3Tag, so thanks for pointing that out; but pipe | seems just cleaner and I cannot confuse it with blanks or spaces or other non-printing characters. I am probably too old fashioned ... and too visually inclined to use TAB 
And I agree with your point about data integrity, and not misusing fields. But please note that the current values of the ALBUM and ARTIST fields are not the standard values from a database or meticulously copied from the CD cover. It is often already shorthand or just partial information, so I don't feel that I violate a high-integrity data field with unstructured data. And in any case, I keep the initial data unchanged in the two ORIG- fields in my tag structure of all songs, so all changes I make are fully reversible (if I want to do that in the future). It is indeed a way of working for convenience and a preference to make it easier to do things in iTunes.