So, just to clarify:

1)Create UnitStat_LightningWeakness in example_coreunitstats.xml, set to 1.

2)Add GameModifier to each 'metal' item which adjusts UnitStat_LightningWeakness by 10%:

<GameModifier InternalName="LightWeakAdjust">

  <ModType>Unit</ModType>

  <Attribute>AdjustUnitStat</Attribute>

  <StrVal>UnitStat_LightningWeakness</StrVal>

  <Value>10</Value>

  <Operator>%<Operator>

</GameModifier

3)Then in the GameModifier "Damage" for spell "UltraMegaLightningSplosion" something like this:

<GameModifier InternalName="UMLSDamage">

  <ModType>Unit</ModType>

  <Attribute>CurHealth</Attribute>

  <Calculate InternalName="AirShardBonus" ValueOwner="CastingUnit">

    <Expression><![CDATA[[UnitStat_NumAirShards] * -5]]></Expression>

  </Calculate>

  <Calculate InternalName="LightningWeaknessBonus" ValueOwner="TargetUnit">

    <Expression><![CDATA[[UnitStat_LightningWeakness] * -50]]></Expression>

  </Calculate>

  <Calculate InternalName="Value">

    <Expression><![CDATA[[AirShardBonus] + [LightningWeaknessBonus]]]></Expression>

  </Calculate>

</GameModifier>

Thanks.  Potentially a lot of work, but I like the possibilities this opens up.