OK, it's now been two years since
the original experiment...
Here's how they look now:
As you can see, the untreated tread plate on the left is still groady... in fact it is even worse in real life than the picture shows.
And on the right is the plate which was treated with BAC, still looking clean, tho grey. I said BAC, but for this experiment I actually used a version of BAC that has some silicone functional groups substituted for some of the carbon-based groups. This version, called "Gold Shield" (sample provided by
Drew - thanks!) is considerably more proof against washing out. In fact it is used by hospitals to sterilize and keep sterile their bedding, even tho it is frequently washed.
I call this experiment concluded, and a success. I am going to treat the other tread plate, and our bare teak rub strakes, with Gold Shield.

A Tale of Two Tread Plates, Update