I've read several threads on Brooks saddles. One person posted his many years of experience with the Brooks brand and added photos. He said he never used anything on the leather. It was impressive how many decades of use he was getting out of these saddles.
If I am using the Yakima Steelhead for transport I use a couple of those plastic grocery bags. They are easy to tie on. . I have a thick freezer zip lock type in the pannier in case it raining and I need to get off the bike.