Wow, the "Corrosion Hypotheses" document is fascinating. It provides a way to find a couple of important pieces of information, namely, what happened to all the missing rivets, and it could date the lone present rivet to a specific era, i.e. pre-WW2, as Ric indicated above.
Seems like the tricky part about that experiment would be finding a supply of rivets from that era.
Might it not be possible to cook up a batch of those low-Mg rivets for the salt-water test? The recipe
has to be around, doesn't it? Or maybe you could use a a donor rivet to determine the composition? One could be borrowed from a pre-war museum plane, or a known wreck in the field.
The story keeps getting more interesting, you can feel it coming to a conclusion, if only in a Zeno's Paradox way.