Actually you're wrong about an AT-AT weak spot on the neck.
The weakest spot
is the legs (thighs to be specific), where the missiles lock onto.
See here in the settings for the AT-AT:
Code:
BFBuilder/Assets/Sides/IMP/ODF/imp_walk_atat.odf
//***********************************************
//*********** DAMAGE VALUES ****************
//***********************************************
DamageStartPercent = 60.0
DamageStopPercent = 20.0
DamageEffect = "vehiclespark"
DamageAttachPoint = "bone_fl_thigh"
DamageStartPercent = 60.0
DamageStopPercent = 20.0
DamageEffect = "vehiclespark"
DamageAttachPoint = "bone_fr_thigh"
DamageStartPercent = 60.0
DamageStopPercent = 20.0
DamageEffect = "vehiclespark"
DamageAttachPoint = "bone_bl_thigh"
DamageStartPercent = 60.0
DamageStopPercent = 20.0
DamageEffect = "vehiclespark"
DamageAttachPoint = "bone_br_thigh"
DamageStartPercent = 45.0
DamageStopPercent = 20.0
DamageEffect = "vehiclespark"
DamageAttachPoint = "bone_fl_calf"
DamageStartPercent = 45.0
DamageStopPercent = 20.0
DamageEffect = "vehiclespark"
DamageAttachPoint = "bone_fr_calf"
DamageStartPercent = 45.0
DamageStopPercent = 20.0
DamageEffect = "vehiclespark"
DamageAttachPoint = "bone_bl_calf"
DamageStartPercent = 45.0
DamageStopPercent = 20.0
DamageEffect = "vehiclespark"
DamageAttachPoint = "bone_br_calf"
As you can see the thighs are where the most percentage of damage is done. The rest of the AT-AT (except the calfs) take the standard amount of damage.