Hello:
Does CourseLab allow placeholders? For example, if I know that I want to insert an audio clip at a certain point during a lesson, but the audio clip is not yet available, can I creat a temporary "dummy" frame or "hole" in the lesson to hold the place where I will soon insert the audio clip?

You could just have a dummy audio file, make a second or twos recording of something or nothing.
Save and name the file as the final one will be and add that to the page or timeline, depending on how you are handling it.
When you have the new file just replace the file in the folder 1 where the initial one is.