I don't know how hard it would be to code this, but how about having mercenary units available for hire?
Pay them a base per turn fee, and increase the fee if the unit becomes damaged. Increase ALL mercenary fees if a merc dies.
If you can't pay them hard cash, (if your treasury would go into the red at the begining of your next turn) you have to select mercs to dismiss.
As an additional feature have the price be based on your alignment to a certain extent. Good empires would obviously be able to keep mercs happier. Maybe a random event that would cause some of your mercs to turn on you, especially if they have been taking heavy casualties.
Perhaps even work it into espionage by being able to tell which units are mercenary, which would make them primary targets (as it would impact you militarily and economicly)