Well, it wouldn't be a study they could do quickly. As someone mentioned earlier in this thread, it would be helpful to have a baseline of the sufferer's condition before the MLC started, as well as after. So you would need to recruit a bunch of married couples at an earlier age, say 30, do tests on them periodically, interview them periodically, then if either party started to show the behavioral symptoms of midlife crisis, you would look for physical manifestations. Follow through until those manifestations pass, see if there is any change in anything. Compare them to people whose behavior never changes. We are talking here about a multi-year if not multi-decade study. And then there would be the issue of what physical aspects would you look at?
The thing is whoever took this on as a research topic would be starting from nothing. Another approach would be to lobby the psychiatric community to get MLC in to the next DSM. No one is going to want to fund research on something that no one even recognizes as an illness to begin with. Once it was recognized, then there would be a reason to research it to look for a cure.