<GameModifier>
            <ModType>City</ModType>
                <Attribute>AdjustDefendingUnit</Attribute>
            <StrVal>UnitStat_MagicResist</StrVal>
            <Value>10</Value>
        </GameModifier>
Nope, that doesn't work either...
At this point, I've pretty much tried every combination of the Attributes I have in the GameModifier above... none of them worked. City Info screen just shows the Magic Resist icon and +1000% from the spell, but does not actually modify the Magic Resistance stat at all, either stationed or trained units.