@snood,
I think the former president address of Mr. President would rightfully prevail. I can't see that anyone could think that Mr. (Male equivalent of First Lady, whatever it is), would be used instead. For one thing, being president is an office one achieves themselves, not as through one's spouse.
Similarly, I would think that if a President, male or female, was married to a former Cabinet secretary, the Mr. or Madam Secretary designation would prevail.