Here's a script which does it.
If anything is selected, it will do a recursive slideshow of just the selected files/folders.
If nothing is selected, it does a recursive slideshow of everything in and below the current folder.
I'll do a more detailed write-up and post this in the Buttons & Scripts area later on.
Edit: This requires 12.9.4 or above.
Edit 2: Moved script to proper post in the Buttons & Scripts area: