I may be talking way off base here, but why couldn't the spawn tables just incorporate a temporal component?  No script that needs to be run to swap the pops, but extend the model to incorporate time of day and you have a much more flexible solution... some mobs could only be up during certain hours of the day. 
 
Dang, now you're gonna make me go look at the spawn code... 
		
	
		
		
		
		
		
		
		
		
		
	
		
		
	
	
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