In Outlook 2003 you can apply labels to calendar appointments so they are more easily identified, but is there a way to apply them to several appointments at once?

Yes. You can do it one of two ways.

1. You can create a custom view that uses automatic formatting to apply labels to appointments based on any criteria you choose. Using a view to apply a label works only while the view is used, the labels are not permanent.
   
2. You can create a group by label view and drag appointments to different groups. If you use this method you need to have one item per label group to start and the labels are permanent, or until you move them to another group.