The main problem is that lightbox is applied to images that are available on the site. There are at least 2 possibilities I think.

1) Very basic (don’t know if it’ll work) – to include images on your post/page but to hide them using CSS class. All those images must also have a rel parameter set to for e.g lightbox[gallery-1]. You must have 1 image visible to be able to open the gallery and leave other thumbnails hidden.

2. Better, but more complex is to load a page that contains image gallery through Ajax request. You’d need 1 image to serve as a gallery featured image. This image would also have to trigger Ajax call to load other gallery images (or a page/post that contains a gallery of images).

Those are just a quick ideas, never tested. Please show us an example if you succeed using those or other methods.

Good luck