Achieving military leaders to build wonders was one of the fun aspects of Civ3 warfare. I wish they would have kept that ability. They could drop the army creation capability since, as it was mentioned, the AI never found a way to exploit this.
But, could you not have a unit upgrade, let us call it 'the generalship upgrade ', where every time you gain an upgrade you can choose that, and when you reach say the 3rd 4th or 5th ugrade the unit turns into a great leader you can use to rush a military wonder or improvement, or research a military tech for free, or provide a defensive bonus to the city it stays in, or stay in a city and the units produced there gain something more than otherwise normal units would not gain?