You could add a type: partial inside the fields.yaml of the model Post in your case.
You won’t be able to position it in the footer though, except maybe with eoms javascript magic, but you can display it as part the form fields flow.
Hi Radek. You can add custom buttons or anything else with override the backend/behaviors/relationcontroller/partials/_manage_form.php. Copy this file to your plugins/your_plugin/controllers/your_controller folder and rename it to _relation_manage_form.php. Modify it as you need…