Per-pixel hit detection is somewhat tough and inefficient, especially if you want to access the pixel data and check for alpha. I'm actually not even sure how to get at the pixel data in UE4. The alternative is to either enable physics meshes on the SpineSkeletonRendererComponent, or use the C++ API to either test against bounding box attachments, see SkeletonBounds spine-runtimes/SkeletonBounds.cpp at 3.8. This could be extended to also test against RegionAttachment and MeshAttachment, but that'd again be a bit inefficient, as one would have to calculate the world position vertices of each attachment.
