Hi @cbirdsong,
I don't know of a way to set that via sis_import, so a course template or a separate API script to crawl through all courses and set that parameter would be my suggestions. Using python, you could do this in a relatively short script and perhaps run it daily (to catch any new courses that may get added). If you wanted to remove the "public" option from the dropdown list in the UI, I think you may have to use a MutationObserver, and faculty could disable javascript if they really intentionally wanted to get the public option back, so I'd probably suggest leaving the permission to change that disabled and do the API script route.
-Chris