The use case reported here sounds like a classroom or lab environment with many people (and likely novices) using open environment machines.  In such cases, if users don't have write access to their home directories, it seems to me that there's no need to try to preserve IDLE configurations across sessions.  Asking the user for another location in such cases would be confusing and add needless complexity.  I'd say either just create a temporary directory or create the config files as temporary files as needed.  For advanced users, I suppose a command line option could be added but has there been any demand for such a feature?