Well, i know that service to show "what we have inside" already are in KSP vanila, but to my mind it are complicticated for design. But i remembered a way how to do this with any item or in one click.
Dunno, are such thingie needed for community?
However, trick is simple enuff. We just need to find a model's transform (any or all) that we need to do translucent, get its renderer, material than - and change its shader on the fly. Of course we need proper shader.
If the need disappears, we need to do reverse operation.
I have not saw someting like that (missed?), so meet X-ray. It can be added like a partModule to any part (or any mesh in part) without any 3D and Unity work.