Yeah, the ability to keep metal in your bones and excrete it through your pores by sheer force of will doesn't really strike me as something that's going to happen any time soon.
Do you happen to excrete fecal matter?
Then you have disproven your own point
The Extremis dose physically remakes the body according to the instructions/specs given to it. It can grow new organs, muscles, tissues, etc.
The poop joke I made above disproves your argument because the digestive system is one of several
biological systems that release/excrete something from within the body when they receive either an autonomic or conscious neural impulse from the brain.
I can't say that it's a great idea to remove a portion of ones bone marrow to store a metal skein, which requires an organic pore (and a fairly BIG one if the art is accurate, around green pea size) for it to come out of, but you could say that about the spatial relationship of the digestive and reproductive systems.
Or the fact that the stomach has to hold a liquid that is constantly
dissolving its walls, or that the intestines have their own taste buds
The only iffy part about the Extremis dose is that the subject can survive
such a radical psyiological change, but that's pretty much the problem in EVERY superhero comic.