If you're zoning something when you load another game (or a scenario, or you 
start a new one), that zoning is still on the mouse cursor. So, you can load 
up the Splitsville scenario, select the Wellspring of Unsavouriness, load up 
your own game at year 0 and place a Wellspring of Unsavouriness! This will 
work for any building that is available in any game (including all Special 
Buildings and probably Omnibolges/Love Domes) - but be warned that if the 
building has a cost, you'll still pay it.