Using shapekeys with Mecanim

Hi, I'm designing a character with a cloak, but I get some glitch during the walk cycle. The legs go throught the cloak. I'm using Mecanim for the character controller.

So I would like to correct the animation of the cloak but I don't know how to do this with Mecanim. With legacy animations, I will correct the animation directly in Blender but I want to learn using Mecanim.

I tried to had some bones to control the cloak and use the transform mask to assign the animation only on the cloak, but it doesn't work. I also tried the skinnedClothes, but I have sharp edges, and there is a bug in Unity in that case.

So is there a way to have a really fine animation with the Mecanim system ? (I use the animations from the tutorial of Mecanim : http://www.youtube.com/watch?v=Xx21y9eJq1U)

Thanks for your Help. Greg