I’d say it very much depends on how advanced you want the function to be.. and user friendly.
I think the way you imagine it could work.. but to make it easier for you you could add a html-field at the end of the repeater row with an a -tag which you hook up your custom script to.. that way you wont have to hack anything into the repeater.
You’ll have to do some custom javascript obviously.. do this by adding a filter to admin_head:
http://stackoverflow.com/questions/3326967/how-to-add-custom-javascript-to-wordpress-admin
With the script I suppose you’ll just have to be able to integrate that script you’ve linked.. then have it output the coordinates into the appropriate fields..
I don’t think you’ll have to publish the post with the image before using the script/popup. You’ll retrieve the image when the user actually click the “add coordinates” popup-link anyway so as long as there’s an image there in the code your fine.
I have to say this is a deucy and it’d be interesting to make it into a plugin.. I haven’t had use for a function like this myself but I’ve seen this being used on occasion.
