I can't give you an answer, but I can give you something... In the code for the Eyes of the Eagle spell is the following code.

<GameModifier InternalName="Reveal">
      <ModType>Map</ModType>
      <Attribute>Reveal</Attribute>
      <Duration>1</Duration>
    </GameModifier>

I wonder if this GameModifier tag could be adapted to an improvement.... I know it's not much, but it might be a starting point. If I still had v1.09, I could do better, but I don't.

I tried:

    <GameModifier>
      <ModType>City</ModType>
      <Attribute>AdjustDefendingUnit</Attribute>
      <Value>25</Value>
      <Operator>+</Operator>
      <StrVal>UnitStat_Sight</StrVal>
      <PerTurn>1</PerTurn>
    </GameModifier>

and it says all units in the city are getting +25 vision but they aren't actually piercing the fog of war. I will try your idea LightofAbraxas, thanks for the help.

Thanks a lot impinc.