Ive never seen anything in cold writing as to the format of a Letter of Intent, but it is definitely a more professional way of doing things than most people would go about. Your asking these people to put pen to paper and take time out to herald your cause. If you know them personally (coworker, family friend, etc) than yeah you can get away with a much more casual and personal way of asking. A lot of guys get hung up on trying to find somebody in a senior Military (primarily Army) leadership position either active duty or retired. Not all of us have friends and family that served and the board will understand that when they go through your packet however if you can get the time to interview with a senior military official who is impressed enough by you to take the time to write something that says a lot too. They cant really speak for your experiences, but they can speak as to your professionalism.
Id say your best bet would be to get with anybody that writes business documents on a regular basis (resumes, memos, etc) because they will offer a far better opinion than somebody like me that got C's in English.
