Thank you very much for the input, Neo!
Oooh, those Warforged feats are very nice! I had never read them before!
On the plates as feats, I just fear severe balancing issues might arise. Mirran creatures are born, not made, and as such all these feats could only be selected at level 1. I could ease the restriction on both Argentskin and Cold Ironhide, making them non-exclusive (i.e.: you have enough traces of cold iron and/or silver in your knuckles to make them overcome DR), which would make them not unlike Silver Tracery and Cold Iron Tracery, but they would need some buffing, as you are spending a/the level 1 feat in something while you could be getting something more interesting, such as Mithralscale or Adamantine-plated. On the other hand, these last two would need serious nerfing: again, they would be level 1 feats, and as such could never have pre-requisites that might ease their power. Also, I can't make them like Eberron's Adamantine Body and Mithral Body as theoretically Mirrans can don armor (though they rarely do ). All that said, I'll give it a try!
Example human fixed!
I also thought about turning them to constructs! Problem is, they are pretty much simple organic creatures with metal covering some parts of their bodies, and there are lots of golems wandering Mirrodin's surface, who are real constructs. Also, I feel like Phyrexians should also be constructs. Better add some diversity :)
I thought about SR too, but at a much higher proportion, which seemed unreasonable to me. Yours is pretty balanced! I'll just have to check if that has any lore to it, though, even if it's a stretch. Anyways, what do you think of something like Warforged's "Light Fortification"? They DO have metal covering their skin, after all, and some of it just might get in the way of critical hits!
Oh, and on Memnarch, I just might give it a try and make it! I'll have to study him a little, though