I've heard of copper reacting with SO2 gas to create copper sulfate, but I haven't seen the same data showing that it reacts with thiols to remove onion flavors.
I have gotten onion from Mosaic before. FWIW, I believe it was from 2013 whole cone hops from freshops. I only ever picked it up in a single hop beer - I've never picked out onion in any other beers I've brewed with that batch.