You use something along the lines of:
Events.EndCombatSim.Add( MyOnCombatEnd );
Whenever a Combat ends, a function you've previously defined as follows:
MyOnCombatEnd( m_AttackerPlayer, m_AttackerUnitID, m_AttackerUnitDamage, m_AttackerFinalUnitDamage, m_AttackerMaxHitPoints, m_DefenderPlayer, m_DefenderUnitID, m_DefenderUnitDamage, m_DefenderFinalUnitDamage, m_DefenderMaxHitPoints )
gets called.
However, as far as I can see, until we actually get a decent Lua API reference, it's a real pain in the neck to do much work in Lua for now, since you've no idea what methods/functions there are usable for different objects beyond the ones you can find in the current code.