How to create animation for health bar? (video)

Here is what I need to recreate: http://screencast.com/t/68OH2u8j

As the amount of resize that must be done is dependent on dynamic conditions (like how much energy is used, damage is dealt ect.), the animation cannot be with exact size change of the bar. The problem is that I could not find a way to create animations with Mecanim that can have properties modified from code. You could use SetCurve of animation clip but it seems super cumbersome and I am not sure if it even creates the desired result.

I could probably easily do it with something like iTween but... it seems strange that I cannot find a way to do this with Mecanim.

Am I missing something?