I don't think the metal plates are hemalurgy because the fused would have been on Braize during the time that Hemalurgy was first introduced on Scadrial and they've only been out for a year - even if the fused have been working with Scadrian worldhoppers or other who are knowledgeable about Hemalurgy, would El have had enough time to learn the very specific mechanics (such as where bindpoints are) of hemalurgy and then modify himself to this extent?
Lezian also notes about El, on page 1204, that "He always ripped off his natural carapace formations at each rebirth, then replaced them with metal inclusions. They were incorporated into his body by Voidlight healing and his own special talents." (I don't have an ebook to do the fancy quote thing with). This is like El's signature thing, and he'd been doing it long before Hemalurgy was even discovered. It sounds to me like he's doing something else with these plates, based on the "special talents" part of the quote. When I first read that section, I wondered if the metal was aluminum, to function as a built-in defense against shardblades.
I saw someone somewhere (I don't recall who or where, unfortunately) musing that the metal inclusions could be like a fabrial cage. What if El's "special abilities" allow him to trap second spren inside his gemheart without it messing with his form, soooort of like Timbre was able to trap Venli's voidspren? Then El could turn himself into some kind of walking fabrial. I suppose the problem with the "trapping lesser spren" theory is that, if I'm remembering correctly, the Fused didn't know about modern Rosharan fabrial mechanics until this Return.
Edit: fixed some errors about the Fused