I haven't done research on a topic like this but I do have a couple of friends now graduating college, starting OCS this summer, and going on to become officers for the Marines. Wouldn't attending a regular university and going onto the marines after be the better option? I say this because technically the main goal of USMMA is have graduates come out as Merchant Mariners to serve the DOT and not Marines. Also considering the fact that they invest that much money into each student they probably expect you to serve the DOT after graduation.
Again, I haven't done extensive research on what you are asking but from what I have talked about with my friends it doesn't make sense from my eyes to attend USMMA and then go onto becoming a Marine officer. If I don't receive an appointment from the USMMA attending a university and applying for the Marines once I graduate will probably be an option I will be exploring.
Hope this helps.