The MS law in question has been cited ad nauseum today on at least half-a-dozen other FR threads, specifically the threads dealing with the Mississippi run-off Tuesday. One of the threads even repeats the MS Code verbatim. But sorry that I didn't bookmark that material.
The cite I saw was to the March 1st deadline, not write-in qualification. The post seemed to indicate that it referred to both. Other than that, there are references to a 3-1/2-year-old article in a Tupelo paper that quoted an SOS employee.