So just as an update, RGM don't have any stock of the SS panels, only plastic.
...... because Covid (again)...
I saw a nice brushed type here, and asked if they did a polished one, but alas no.
https://www.tuning-art.com/en/load-edge-protection/fuer-audi/stainless-steel-bumper-protector-for-audi-a4-avant-b8-fl-12-2011/a-10014
So, I'm now filtering through some eBay types. I'll keep you posted.