Get post offset/posts page offset in single post page (outside the loop)

I don’t think that you can get an offset of the page outside of the loop, since it will be out of the context. Offset may vary depending on the arguments you are using to get posts for the loop.

Easiest solution will be to calculate offset in the app. You will need to pass post_id as an attribute and then loop through all the posts obtained by ?json=get_posts&orderby=date&order=desc&count=x&page=n and compare their IDs to the one you got as an attribute.

Hope it makes sense.