Plating was changed to block movement and take up space like most other parts as a simpler alternative to the old system - parts were randomly flagged "NO_REINFORCE" (without much thought - you could reinforce a seat, but not a kitchen unit) and you had no way to know this except trial and error (or reading the json files).
It also didn't work, because while you couldn't install plating over a NO_REINFORCE part, you could put the part over the plating and that was allowed. Putting armor in its own slot would necessitate the return of the NO_REINFORCE flag.
An armored bumper is a more reasonable way to look at it, particularly since there are flavors like spiked plating available that are supposed to collide with zombies before other stuff does.
They could easily be made opaque, but that would make them inferior to boards. Not blocking movement (much like armoring a frame in the old version) helps with collisions, but not with zombies trying to break inside.
As for wheels in an empty space: nope. Everything except spikes/blades needs a frame first.