http://developers.facebook.com/docs/reference/api/post/
In Comments -> Create section it says:
“You can write to the POST_ID/comments connection to post a comment to the post by issuing an HTTP POST request with the publish_stream permission and following parameters.”
So its possible to post comment on any post through the app if you have publish_stream permission via access_token?
Yes, on any public post, or post visible to the user. You should ensure any text in the comment has been entered by the user though – not automated.