The issue here: what holds these plates in place?
Metal pieces are routinely used to reinforce / augment / substitute for bone. Plates to replace pieces of skull are the type that comes to mind. Here is an image of a man who has a plate instead of the front of his skull.
This works because the metal is firmly affixed to bone and the bone is immobile. The bone has over it not only the skin, but all the other subcutaneous structures: nerves, vessels, fat, muscle. The metal augments the bone but does not protect the rest.
A problem with having the metal unaffixed to bone is that it will not stay in place. Most people who have loose pieces of metal in the body are soldiers who have been hit by shrapnel. My mom tells stories of a WW1 vet uncle who would have his wife pick out pieces from his back as they emerge. Googling it up I found this witty and horrifying blog from an Israeli terror bomb survivor.
But enough of the bad stuff, now it is time for what makes shrapnel
fun. After it goes in (not the fun part), it comes out! All by itself!
What I have learned is that shrapnel often slowly but surely works its
way up to the surface and is expelled from the body. Every day I check
my body for objects which, like lounge lizards slinking out late at
night from a singles event, are starting to emerge. I then do the
I examine the item, and try to guess what it is. Metal? Glass?
Plastic? I brush it gently with my fingers, to see if it will
dislodge. If it does, and it isn’t really, really teensy-weensy and
non-impressive, and if it doesn’t fall from my finger onto the floor
and get lost, I put it into my “Official Machane Yehuda Bombing
Shrapnel Collection Test-Tube”. If it doesn’t dislodge, I gently feel
the area around the shrapnel to check for swelling, edges, etc. This
gives me some indication as to the size of the piece, and whether it
is going to require medical assistance to remove. Size and/or swelling
be damned, I try to remove the item myself. I jiggle it a bit, push
around it like you do with splinters and try to pull it out with my
eyebrow tweezers. I smack myself on the hand and tell myself to stop
playing with the shrapnel and to let it come out on its own. Bad BAD
Gila!!!!! If my cooler friends are around (cooler being defined as
anyone who find this whole process fascinating as opposed to
disgusting”), I call them over, and show them. If no friends are
present, I make a mental note to show them the next time I see them. I
put a glop of iodine ointment on the area and cover it with gauze and
tape. The combination of iodine ointment, gauze and tape is wonderful,
and has become my standard medical treatment for just about
everything. Every day is a new adventure as I find all sorts of
foreign objects emerging from my body.
I think this would be the fate of your augmented soldier with metal under the skin. The metal would migrate around and work its way out.
Last: the prospect of using an unanchored piece of metal as armor. What happens if this metal gets hit? Will it not be driven by the force deeper into the body? These are not large pieces and so will not meet much resistance from deeper structures.