I would like to add this to Invoice as well, but I'll wait for that.
Once thing I did want to mention was that I preferred to have this new Admin page under Customers instead of Tools. So, I put this line:
'<a href="' . tep_href_link(FILENAME_EXTRA_FIELDS) . '" class="menuBoxContentLink">' . BOX_TOOLS_EXTRA_FIELDS_MANAGER . '</a><br>'.
Into /admin/includes/boxes/customer.php (between Customers and Orders) instead of in /admin/includes/boxes/tools.php
I realize that I should change it from BOX_TOOLS_.... to BOX_CUSTOMERS_...., but I'm feeling lazy.
I also modified the line in /admin/includes/languages/english.php from:
define('BOX_TOOLS_EXTRA_FIELDS_MANAGER','Extra fields manager');
to:
define('BOX_TOOLS_EXTRA_FIELDS_MANAGER','Extra Fields');
so that it would fit in my left column without wrapping. A small thing again, but I'm a bit AR. ;)