As far as I am aware, there are currently no plans to add support for sharing on Instagram.

If you wanted to add support for a social media platform which is not currently supported by Juicebox, then you might be able to implement it using the Juicebox-Pro API.
As long as the social media platform has its own sharing functionality (where you can send information to their web site via a share URL), then you could manually add an HTML button to your web page (alongside your gallery) which, when clicked, would fetch the required information about the image currently being displayed in the gallery (using the available Juicebox-Pro API methods such as getImageInfo()) and use this information to populate the parameters in the share URL.
Knowledge of HTML and JavaScript would be required and while this may be possible, it might not be easy to achieve (although I hope it points you in the right direction if you plan to try it).