I have modded a new Attrition:

units and heroes in non-allied regions will lose 5% of health each turn.

I have seen many post about this idea, but no mod seems to have done it.

The trick was to allow regen values to be negative ( in SimulationDescriptors[EmpireType]):

      <!-- Unit health Regen-->
      <SimulationPropertyDescriptor Name="GlobalUnitRegenImprovement"          MinValue="Negative" BaseValue="0.05"/>
      <SimulationPropertyDescriptor Name="InAlliedRegionUnitRegenModifier"     MinValue="Negative" BaseValue="0"/>
      <SimulationPropertyDescriptor Name="InOwnedRegionUnitRegenModifier"      MinValue="Negative" BaseValue="0"/>
      <SimulationPropertyDescriptor Name="InNoneAlliedRegionUnitRegenModifier" MinValue="Negative" BaseValue="-0.10"/>

Another effect of this, will be that the winter effects that reduce regen, will also be able to go negative...