Why not in ACF.
The goal and scope of ACF from the beginning is to allow the creation of fields management in the admin that anyone could accomplish using built in WP functions for meta boxes (custom fields) allowing us to make this part of the development process easier and faster.
To this end ACF uses a standard WP_Query() to get the posts that will be listed in relationship and post object fields.
ACF also supplies a plethora of hooks for filters and actions that allow extending it for those where the functionality that 90% of users is not enough for other 10% of us.
Beyond this I can only suggest contacting the developers.
