I don't know of any plugin other than the one you mentioned. Maybe since you posted in the developer forum you are open to trying to make your own?
I've heard of (but not used) the Helix Toolkit that should let you import an STL and presumably get the surface data out of it. You'd then have to write code to re-create these surfaces in SE, and if possible stitch them together to create a solid body. If you got as far as being able to translate the STL triangles into SE surfaces, you could just use normal SE tools to stitch them together.