I've never heard of it. Mrs. and its various forms all have their own awkwardnesses, but I'm sure something will be worked out.

I don't think Rev. or Dr. would be added to "President," seeing as they're rather small accomplishment compared to being elected to lead a country. Plus division of church and state, so I for one wouldn't feel comfortable having someone be addressed as Rev. President. At this point, even if the President was a doctor or reverend, or anything else that might have its own special title, the title would stay "Mr. President," simply to maintain continuity and keep things from getting confusing. At least I hope so, because that makes more sense than anything else to me.

And it would likely be "First Gentleman." They do say, "Ladies and Gentlemen," after all. First Lord sounds strange, First Man sounds weirder, but First Gentleman just sounds a bit awkward to say due to its length, but otherwise is perfectly acceptable.