I think the way S.Hussein did it was that he centered a location on the larvae, not on the hives. So basically, it would go something like this:
Conditions:
Player x brings at least 1 larvae to "anywhere."
Actions:
Center location "whatever" on larvae at "anywhere."
Conditions:
Player x brings at least 1 larvae to location "whatever."
Player x brings at most (or exactly, whichever you prefer) 0 Lair to location "whatever."
Player x brings at most 0 Hive to location "whatever."
Actions:
Create 1 Slow Zombie (or whatever) at location "whatever"
Remove 1 larvae at location "whatever" for player x.
Conditions:
Player x brings at least 1 larvae to location "whatever."
Player x brings at least 1 Lair to location "whatever."
Player x brings at most 0 Hive to location "whatever."
Actions:
Create 1 Rage-Infected Human (or whatever) at location "whatever"
Remove 1 larvae at location "whatever" for player x.
Conditions:
Player x brings at least 1 larvae to location "whatever."
Player x brings at most 0 Lair to location "whatever."
Player x brings at least 1 Hive to location "whatever."
Actions:
Create 1 Crimson Head Zombie (or whatever) at location "whatever"
Remove 1 larvae at location "whatever" for player x.
To make it so that other buildings affect what unit you get, just add in a condition that requires you to have the building in location "whatever" and change the unit to whatever (make sure to add "at most 0" conditions for each building, or else it'll spawn both the new unit outcome and the original).
For example:
Conditions:
Player x brings at least 1 larvae to location "whatever."
Player x brings at most (or exactly, whichever you prefer) 0 Lair to location "whatever."
Player x brings at most 0 Hive to location "whatever."
Plyaer x brings at least 1 ultra den to location "whatever."
Actions:
Create 1 Suicidal Zombie at location "whatever"
Remove 1 larvae at location "whatever" for player x.
(Note: The disadvantage of this system is that if you build a building or whatever by the hive and the larvae decides to come out on the other side of the hive/lair/hatchery, ie, you build a hydra den on the top side of the hive and the larvae spawn at the bottom, the building will have no effect =/. I supposed this would be solved by making the location larger, however, make sure it doesn't get too large, since you don't want one hive to be able to upgrade a bunch of hatcheries horded around it or something like that)
PS: Make your sequel less rigged than S.Hussein's original =P. Zombies Europa was one of the few maps I've seen to have been rigged for
both sides XD. Ownage hero mutalisks for the zombies, super-armor fbats for the Germans, MIGs for the russians (seriously, hydras can't even touch MIGs XD), etc.
Also, make sure you get copyrights and such from S.Hussein.
None.