Editing the master of a single slide in Keynote

In keynote, I know how to edit the theme and create a new master slide. (By going into View -> Show Master Slides.) However, I would like to edit the master of a single slide.

Here is an example of why I want to do this:

Is there some way I can unlock an element of the current slide's master (i.e. that horizontal line), without editing the master itself for other slides?

It is the purpose of masters to make all slides look the same - you cannot ask to have masters and differently looking slides at the same time.

Copy the master, modify it and assign that one slide to the modified master.

1. Create a borderless white rectangle and cover the line or part of it with the rectangle. Move the rectangle behind the text.
2. Put the text into a filled rectangle and cover the master line.