i think you already said the best way to do this - have a duplicated skeletal mesh in your character blueprint thats either been scaled up slightly before import or has a displacement in the World Position Offset along the vertex normal, and then just trigger it on when the shield is active.
It wouldnt be too expensive to have a single layer of transparency (less still if you used additive blending) and you can set visibility of the mesh to off before and after the shield effects turns on and off so it wouldnt always be rendering.
you could potentially use the custom depth buffer and create some sort of post process but i think the result would look way worse than just duplicating the mesh. (Multi-color Outline Post Process - Tom Looman)
not sure about rendering the same triangle twice with different materials - sounds like something that would require custom code and probably wouldnt be much cheaper than just duplication.
There’s a material option called Crack Free Displacement, could try that - not really used displacement all that much tbh. I would have thought most use cases for displacement would be on a smoothed mesh anyway though. Could always add a support loop and paint the hard edge verts black and use vertex colour to scale the displacement so they dont move, depends what sort of effect you’re after.