I first supposed
Quick Preferences does exactly this job: Quick Change Preferences ;-) and not only Change the import settings ^^
The Idea with the start-script is not a bad one, thus, i prefer when it's in Calibre and not an own solution, because some very comfortable things won't work any more (p.ex. quick change of Libraries)
Maybe a doable way for all is allowing configuration also as a per library setting. If in the Library folder is a config file take this settings, else use (default) user settings...