In a form to view a single record, it’s possible to display fields from connected objects. But in a form to edit a record, you can’t. I’m guessing this is connected to the inability to set a field to non-editable. I hope it’s something you can add.
Sorry, the title of this request is wrong. It’s not about variables. (But I might have something to say about that later. Unless I can manage to pass all the values I need through utility fields.)