Here's my opinion. Anyone who would try to do this has serious hangups and obsessions with having an MD after their name. I think it would be a waste of time to want to go back to school or residency when you could be spending your time doing what you apparently came to school for: practicing medicine. I mean, what is your real goal in life if you want to go back and get an MD? If it's that important, I would advise you to buy a white coat, and have your name and the M.D. monogrammed on the coat.
That said, I've never heard of this happening, and if I were on an admissions committee, I would drop that person's application without a second thought. I take that back. I would get your hopes up and invite you for an interview, just to be able to ask you what the hell was going through your mind, and then drop your application. Actually, by trying to do this, I would think you would not only be sh*tting all over osteopathic medicine, but on medicine in general.
The person who would want to go for an MD after already having obtained the DO would seriously need to reassess their priorities and their seemingly conditional commitment to medicine.