Is it possible to add custom fields & labels to the back end of zen photo. I would like to use zenphoto to display artworks. Perhaps I could repurpose the existing fields? Or add new ones?

For example:
Title
Description
Maker
Maker Biography
Materials
Measurements
Date Made
Credit Line

Is this a ridiculous question?

You can't do that without hacking the core what we clearly don't recommend regarding updates.

As you probably noticed title and description are always there anyway... Also there already enough fields like "location" and "city" you can use for that. How you call the output of that fields is only depend on your theme's layout.

Also you can enter all those info directly in the description field, too.

Of course, there is also the custom data field which is intended for this sort of application. Just store your data in that field separated by some kind of separator. Then you can extract it using the PHP explode() function. I think if you search this forum for topics like custom fields you will find lots of discource on this.