One other aspect of this may be the confusion that ensues when changing the setting doesn't change the prompt when you activate it.

It would be possible (though not necessarily trivial) to update the activate scripts to read the prompt from the file, though I don't think it's necessary. But we'll get a bug report sooner or later anyway.

Maybe if the setting is called "provided-custom-prompt" that will imply enough that it's a optional record of the prompt rather than an active configuration setting?