Well, I think I have to report - confess - that I think this is the result of water leaking in from the coolant system, via an improper gasket and my insufficient cleaning out of the old oil when doing an oil change.
The last 2 oil changes (at about 100 hours each, which comes out to about once a year) I have treated an oil change like a car oil change: pull the plug, wait, put plug back in, refill.
In this investigation, I went all the way down to the white paint: I was in there with fingers and rags until every drop was out. The sludge that I had previously not cleaned out was pancake batter thick, at best. I did get all the way down and found a magnet in there (did I put that there, or is that standard??) but there was nothing stuck to it. So, previous changes didn't do half the job it should have.
So, I think the silver IS in fact metallic powder of wear and tear, but caught up in water-infused oil.
Still, nothing seems overly loose or worn, surprisingly. This summer, I'll put the head again and attempt another gasket.