Hi PhoenixLament, this is theoretically possible but you’d have to take a completely different approach. Be aware that this is far from the “easy” that this plugin was intended for but here are the basic steps:
First install the plugin Show Content Only https://www.ads-software.com/plugins/show-content-only/ and test if you can get an image attachment page to show only the content without header, sidebar or footer by appending &content-only=1 to the page URL
Then, if that works (I never tried) you will need to activate the iFrame option on Settings > Media.
Finally, create a link in a post that uses the attachment page as target and give it the attribute class="fancybox-iframe" so it will open in FancyBox.
Finally test and tinker to make it fit your theme and usual image sizes.
Let me know how it works out and share a link if you need help ??
