Unfortunately, JuiceboxBuilder-Pro is able to extract only the IPTC Document Title (for use as the image title) and the IPTC Description (as the image caption). (Just go to 'Images -> Titles -> Use IPTC Title' and 'Images -> Captions -> Use IPTC Caption' from the drop-down menu at the top.)

Currently the only automated way to extract any other embedded metadata from images would be to use the Juicebox Lightroom Plugin.
If you are an Adobe Lightroom user, you can use the Juicebox Lightroom plugin to create your Juicebox galleries.
Lightroom has the ability to extract whatever metadata you like from an image (EXIF or IPTC) and the Juicebox Lightroom Plugin can make use of this for the image titles and captions.
Once the plugin has been installed, you can select the Juicebox web layout, scroll down to the 'Image Info' section, choose 'Edit...' from the drop-down menu (for the image titles and/or captions) and you'll find the functionality for extracting metadata there.

You can certainly have some custom text in your Juicebox-Pro galleries.
If you wanted some text for the gallery as a whole, then you could use the galleryTitle (in JuiceboxBuilder-Pro's 'Customize -> Lite' section). You could position the Gallery Title using the galleryTitlePosition ('Customize -> General) and, if you like, you could even style the Gallery Title using HTML formatting as noted in this FAQ:
How do I add HTML formatting to the Gallery Title or Back Button?

If you wanted some text for each individual image, then you could add image titles and captions.
Click a thumbnail image on the 'Images' tab and the input boxes for the image titles and captions will be revealed. You can enter custom text there (or go to 'Images -> Titles' and 'Images -> Captions' from the drop-down menu at the top to populate the image titles and captions with preset data (such as the image filenames).

For reference, a list of all Caption Options can be found here.
Just for the record, you could also use HTML formatting for image titles and captions:
How do I add HTML formatting to image captions and titles?

If you like, you can post ideas for future versions in the Feature Requests forum thread.
This keeps all the ideas together and ensures that they are not overlooked by the developers.
I do not know the likelihood of any suggestions being implemented but this is certainly the best place for all ideas.
Thank you.

Edit:

It looks like you've figured out what you were looking to do as I was typing.
That's good to hear! Thank you for letting me know.