Assign only selected polygons

[Viele]

This may require a major rewrite of the core so I am not sure it will work

[alfsici]

So far i am using an "hack" .. duplicate mesh , delete all history and dependencies, un-parent it also, to avoid hierarchies issues, then wrap deform it to original mesh.
Perhaps there might be a way to create a "temporary mesh" out of a selection of polys, using maya api directly, then that could be used instead of hacking it.
I am trowing just a few thoughts your way here, alto i do agree, having the full mesh sometimes gets quite confusing especially if the mesh is very detailed.
Is the onion layer respecting "double/single sided" , i noticed that single sided sometimes is better , by only seeing the front of a mesh, rather then both sides.
Also another option could be a curve outline (perhaps using the maya toon to create a mesh outline first)

[Viele]

That might work as well.
I thought it might work if i add sets that contain components, but for some reason it still displays the whole geometry. I mean maya has the isolate select button, which creates a temporary set. So it should work somehow.
I think the onion layer is respecting double/single sided but it depends on what your viewport setting is. Though I did not test that explicitly

[Viele]

If the main problem was readability, this might be resolved.
I'll leave this open for now, but production testing with the new color and outline options needs to be done.