I can see why you would say that, but I don't think that is the answer in this case. I think the reason you are thinking of a cloak is to add visual width, especially to the mid section. On a normal Necron the Guass gun is the visual point. Given the stark contrast of colours, and the positioning of this colour, the skinny nature can perhaps be over looked. At present we do not see that so the eye is drawn to the thin bottom/middle and the size of the upper torso. In short (just like that guy at the gym who works out in front of the mirror and buys T-Shirts two sizes too small), he's unbalanced and he's got chicken legs.
I think a paint job may fix this. The paint job you described would keep the focus on the top of the model. The other thing would be the base. What are you planning? I would assume sand. Perhaps ensure that all the models have a small rock, a small shrub, hell - even spare nid bits as you probably have plenty. Anything really to catch the eye for when people look down.
A cloak would likely fix this as well. That said we have several things to think of. The colour? What sort of contrast would you offer. Red screams to mind. A dark red underneath and a brighter one for the top. This is great as the adornment, designs and differentiations that could be mustered on a cloak could really elevate the character models. Alternatively a darker cloak could emphasis the eyes or weapons all the more.
Then you have the cost and time. Where do these cloaks come room? Are you buying them? Are you making them? How good are your green stuff skills? Ask yourself, could I do it? Me? Probably, but add a week or two to every squad (and remember how fast I can paint!!).
I don't think it's worth it. At least not yet. Paint up the prototype and ensure it is based fully for your chosen design. Then we'll sit down and figure it out.
Sorry for the rant, but I have high hopes for this army!