It would not make a difference to the sales. The people that would go, would go regardless, and the ones who wouldn't go, aren't gonna be swayed by the old rhythm guitar and the dude from celebrity rehab.
It's a results driven business, with the added pressure of being the first. If it goes well, she'll be a trailblazer, knocking down doors. But if it goes badly, it could set the cause back years.
Playing for Rihanna wouldn't be a struggle for any pro player. Even if yer ain't familiar, you sit with the songs for a few hours and figure it out. No sweat.
The thing I noticed with RHCP is they call anything they noodle for a while a song. Then Anthony raps some stream of consciousness over top, do that a bunch of times and it's another album.
The Chili Peppers with Californication.
Iron Maiden with Brave New World.
Those two come to mind because both bands star had faded quite a bit by the time those came out, but they got relaunched into the limelight on the back of those releases.