To reproduce:
1. Open testPlugin project
2. Open testBlendShapes scene
3. Press play
4. Move around sliders

Expected result: Mesh collider updates correctly
Actual result: Mesh collider stays the same

Resolved by design: The SkinnedMeshRenderer is, as it's name applies, a Renderer. It applies some deformations to the mesh data as it is rendering. However, it does not modify the original mesh asset. For that reason, other components using that Mesh (such as a MeshCollider) will not see any changes on the mesh from the skinning.

It is possible to achieve what you want using the BakeMesh method on the SkinnedMeshRenderer, which stores the skinning results into a Mesh asset:

http://docs.unity3d.com/ScriptReference/SkinnedMeshRenderer.BakeMesh.html

However, I would suggest looking for another way to solve the problem you are trying to solve, as, even if this works, updating MeshCollider meshes in real time is pretty slow, as the physics engine needs to rebake collision data it handles for each MeshCollider each time the mesh changes, which is a slow process.