When I create a post object field it lists the different post types with the posts in the post type below them and sub pages are shown in a hierarchical fashion, intended and preceded by a -, similar to the way WP shows sub pages in the admin list.
Currently I don’t think there are filters that let you alter what is displayed in the drop down, but that would be the way to go. I will mark this post for the developer to look at if he has time and maybe he’ll think about adding something.
