Oasis, of course, already does that.
I guess I don't expect it for the PW, Voyage, or basic Kindle. 
Beat me to it. (Not hard, I started to answer, then got distracted. Squirrel!) If Amazon is considering it as a feature that sets the Oasis apart, no, it probably won't be migrated down, at least not anytime soon.
On the other hand, can't you already switch to landscape mode on other devices? It doesn't "flip" based on how you're holding it, but you can select it, I think. So it wouldn't take that much to allow one to select a 180 degree orientation, and would have value, I think, both for people with simple preferences and as an accessibility issue for people with handicaps.
As always, recommend that people who desire the feature provide Amazon feedback!
Betsy