Layouts with layers

For a Slide Master Layout, you can add a layer (or layers), and add triggers to show those layers. 

However, when you're editing a slide with such a layout, you don't have access to the layer(s) in the Master Layout. 

Here's what I suggest you do: 

• Create a Master Layout to suit "the vast majority of slides in the course." 
• Add a new slide with that layout. 
  • Enter "Set-up Slide" as the title
  • Enter ipsum-lorem or other bogus text (e.g., "text here") in the other placeholders.
• Add the layer(s) you'll need for most of the slides
  • Add the text boxes and/or shapes that will likely be used. Again, just use bogus text.)
• Add the appropriate triggers.
• Duplicate that slide as needed to create the slides in your course. 

TIP: I find it convenient to create an extra scene called "Storage" and put my set-up slide(s) there. That scene can be deleted before publishing, or kept for future use (since it's not linked to the actual course content).