Sorry to bother on a question already answered but after hearing your solution I had to do some experimentation because your solution was quite contrary to what I’ve heard from others in that there was no way to export shape key animation in to UE4. Doing a similar process I was able to import shape key animations into UE4.
What I did was I use a bone in the armature of the skeletal mesh as a driver for one of the shape keys of the mesh. In order to export the morph target curve you have to export the the mesh along with armature when exporting animations. Also Driver bones do not have to be deform bones.
All this is however with the 7.4 FBX exporter in the newest version blender which has shown great improvement in it’s compatibility with UE4.