How to add an outline effect to a mesh that uses a translucent material?

In order to achieve an outline effect around some meshes we make use of custom depth rendering (via the “Outliner” from the UE4 Marketplace). This works fine, however, we are experiencing some problems when applying such an outline effect to a mesh that has a translucent material set. When the opacity value of that material is set to a low value (something below 0.35) the outline effect is not visible anymore.

Now I have to questions:

  1. How can I add such an outline effect to a completely transparent mesh? Can I not use a material with opacity set to 0.0 in this case?
  2. Why does this work when the material is semi-transparent, like with an opacity of 0.5?

Thanks in advance!