Just to say, I still have not worked on geometry caching, morphs, or actual envelopes. Just.. rigs with child meshes for mechanical models.
Here are some tips and tricks.
The nulls of your shadow rig must have it's Center (Origin) unmodified. Do not freeze or reset it's neutral pose. Just move the shadow nulls to where they need to be. The FBX file will create new "Nuetral Pose" nulls in your shadow rig on export otherwise - and I could not make SI stop doing this.. This will confuse the animation or something, and it doesn't import the rig proporly very well. (In fact, it imports each branch of the tree as a seperate skelaton actor making a huge mess in Unreal). Apparently FBX does not have "Local Transform" and works on a 0,0,0 actor center with animation "local" offsets. For the life of me, I could not get Unreal to import anything correctly otherwise.
Plot your constrained animation on the shadow rig before export. Your shadow rig will have each null pose constrained to your IK/FX deformers of your autogenerated rig or to the chains or rigging elements directly. But that animation will not export, nor the constraints, nor the advanced rig you have built. Thus, you have to plot your constrained animation, and optionally (destructively) remove those constraints for export. If you don't remove the constraints, you will get an error - but don't fret! It should still import correctly.
You can only have one animation per skelaton actor. To add more animations, export the FBX again with the new ploted animation. On FBX import in Unreal, uncheck "Import mesh" and select which skelaton actor you'd like to import the animation to in the FBX import drop down dialogue. Apparently only some software not made by Autodesk export to their propriety format with multiple animation libraries within the file. I could be wrong; but if I am right, I'm bemused.