How to format the value of a multiple-value CCK field?

I've defined a new CCK field for my project, which manages a hierarchical taxonomy of world regions and countries: "The Americas" contains "North America", which contains (among other things:) "United States", and so on. All is basically well, but the only display I can get of the (usually multiple) values assigned to the field is to print each country name on a separate line -- this seems to be how the module_field_formatter function is meant to work. What I want is to create something like "The Americas: North America, Canada; Western Europe: France, Italy". But to do that I somehow have to catch the request to print the values for the field AS A WHOLE, and module_field_formatter() only gives me control over the individual items.

So: Is there a way to catch this higher-level request and do what I want? (Note that I'm still back on Drupal 5, if that matters.) Thanks!