True damage would fuck with a lot of champions though.
One of the strategies with say, Alistar, is the fact that he can hard dive you at level six because of the flat damage reduction on his ult. True damage would bypass this.
Instead, I think scaling % armor pen would would better. Starts at 20%, increases by 10% every 10 minutes or something, to a max of say 80%. Or whatever. Change the numbers around, but the effect remains the same. You DO need to keep them from getting too powerful though; a tanky team should be able to dive late game and not get destroyed by the tower. It should definately be a deciding factor that can push the tide or cause hesitation, but it shouldn't be an auto win.