Aluminum rivnuts are plenty strong enough for hanging our non-structural panels. In the real world, I have yet to hear of an FFR falling apart due to corrosion. So unless you live next door to Spongebob, it is not going to matter if you use AL or steel. I do live a the beach, and rust / corrosion happen quickly. I used aluminum myself.

A little off topic:
A word or two on stainless fasteners. I see some guys going stainless crazy thinking that stainless is just some magic kind of steel that doesn't rust. Don't use stainless fasteners where high strength is required. A general ballpark is that they are about 1/2 - 1/3 the strength of a grade 8 fastener. Keep that in mind when going all shiny shiny.

The big PITA with stainless fasteners is galling. If it doesn't have loc-tite, it should have anti-seize. That is a good rule for pretty much anything on a car.