After some investigation, what we found about mobile outline in UE4 is as follows.
1. Hard to implement as PP because no scene texture support in mobile PP.
2. Hard to implement as drawing-twice because no support of front-face culling.
3. Most people implement it as rim-light (or Fresnel) way. But it has limited quality (polygon size and normals affect line quality).
Today we could achieve the following result. It is basically based on the drawing-twice method.
The implementation is now included in our All Platform Outline Pack, and also in our Cartoon Rendering Pack. (written on Jan. 04 2017)