Hi There,
Alought I am unfamiliar with fancybox functionality you could use X Lightbox, maybe the same sytem can be applied to fancybox.
If you would apply X Lightbox functionality to a button on revolution slider, that is how you would do:
Access the page where you would be placing Revolution Slider and edit with Cornerstone.
1 - Add a Raw Code Element and add your lightbox selector, for example:
[lightbox selector=".folio-1"]
2 - You will than create a section and add all images you want to show on your lightbox, except 1 of them.
So you will add classic image elements, and add the class folio-1 to each image.
You can than click on the customize button of the section and hide it on all screen widths. So your images are there, but not visible.
3- You will go to Revolution Slider, add a text element and add the button shortcode with the launcher class folio-1 which you will link to the image which you want to show but you have not added on the page itself.
[button class="folio-1" href="http://example.com/image-1.jpg" ]
The button wont render on Revolution slider but it will render on the front end, once you click on it a lightbox with all the images will open.
If you need more information about the button shortcode, please refer to:
http://demo.theme.co/integrity-1/shortcodes/buttons/
Hope that helps with the implementation of the feature you are looking for.
Thank you