I know you're looking to get at shares through the post_id, but can you settle for finding shares by page_id instead of post_id?
If you can, then you can use the following FQL to get the data you're looking for.
SELECT via_id, created_time, owner_comment
FROM link
WHERE owner = me()
You can then compare via_id against the posts author (page ID) to determine if this share came from the post author in question.
Unfortunately, there seems to be a bug with the data return where some of the via_ids come back as 0. There is a ticket in with Facebook which, at the time of this post, has been open for three weeks at medium priority. I have no idea if this issue is specific to my app or affects everyone, but that query might get you what you want.