A thought that popped to mind is to have a repeater attached to the Image itself… with sub fields of type: Post Object and Text
Then… when you add/edit an image from within the WordPress Media Library, you can create a new repeater row for each Post that you are attaching the image to.
Then… you will always know the Image ID, for which you can look up the Post ID (from the repeater), for which you can look up the custom caption.
Let me know if that seems like a reasonable solution, and if you need further guidance.
Keith
