A quick update:
A couple of days ago, I solved the issue I had with the save-file window in creating the UI elements needed for our naming convention.
If you (or anyone else struggling with this) are willing to put the effort in, go ahead and add a UI element to the file
python\tk_multi_workfiles\ui\file_save_form.py (I basically copied the name_edit/name_label and adjusted the settings I needed).
you find the corresponding functionality for the UI to work. Again I copied the code used for the name_edit and name_label and fixed every bug that I encountered until it worked.
Than the only thing left to do is to create the field-key in the core’s template.yml and add it to the paths that need it.
Good luck and enjoy the path-preview (It was a great relieve for me, when it had finally shown the path rather than the exception).