You are incorrect, drunkensaint. Walls are not part of any zone. The rules on pages 17 and 18 are very clear that a wall is not in any zone. It describes how ranged and melee attacks are still able to target them despite this, but zone attacks and zone effects can not.
Goblin builder's ability as currently written very clearly limits it from affecting walls. It can only use its ability on corporeal conjurations in its zone, and walls are not in any zone.
Arcane Wonders representatives have stated this is an oversight and may change it in the future, but for now the rules prevent it.