Ah! It helps to provide such info in the original question, Jack's Mac. Saves everyone a bit of time and gets you your answer a little quicker! 😉
Generally the more relevant information you can provide in a question, especially if you want to do something without using the normal approach, the better.
I'm not aware of any way of actually changing mp3 tag info directly in the Finder , but there are various utilities available that will allow you to do so without using iTunes. see
http://mactags.blogspot.com/ for a useful listing.
Most audio editing software capable of dealing with mp3's (Audacity, Sound Studio, etc) will allow you to do this too, but will be clumsier to use than a specific tag editor.
Another useful little "add on" is a small app called
"MP3-Info" that integrates with some other "tag renaming" and organising software ( "ID3x", "The Tagger" and "Media Rage" ) or with the author's own
"CDFinder" software, and provides a floating window over the finder for direct access.
Cheers
Rod