Stainless steel exterior doors are seen from time to time. There was a dealer that was either making them or had them made for a while, but he's out of the business.
It's the type of thing to keep you eye open for on the Classifieds here, and on *baY, for example: they do exist.
However, I do not think that SS will go to the same high polish as aluminum will though, will it?
If it's to be, it's up to me.