Would it be possible to add a "New Folder" option to the UI for "Master File" location dialogue box when selecting the render output location?
It looks like a workaround is to return to the Mastering main page and add a /NewFolder/name.atmos to the path.
While the button isn't there the functionality to add a New Folder is still available via KeyCommand. Shift+command+N will bring up the New Folder box.
Didn't think to try that, thanks!