I actually ended up going in a different direction. Since I only needed one post to be the Related post, I changed the field type from Relationship to Post Object. From here, when the page loads with the aforementioned query tags, I use javascript to set the value of #acf-field_<number>-input field to the post_id of the parent. Then, when saving, it properly sets the parent post.
This is a bit hackish, but does what I need it to.
