Correct.
The post object field does have filters that can be used to alter the query used to populate the selections. But in may or may not work in your case. I have not tried this, and the odds may not be good for getting it working. I’ll explain why below.
http://www.advancedcustomfields.com/resources/acf-fields-post_object-query/
you could add a filter that gets the values from the option and then adds a post__in argument to the query.
Why this may not work. There is a function in ACF that runs after the query that groups the posts by post_type, taxonomy and parent posts. When this function runs if there are hierarchical post types in the mix and the parent post of the post that you want to show is not also in the group then there’s a good chance that the post you want will not appear.
