I know a few guys doing it but they are just rolling the dice, hoping they will not get caught. None of the airline they work for asked for any letters, they just said they were taking mil leave and were not too specific with what the duties were. If asked for letter, I am very sure they could find someone to sign a letter stating they were performing military duties...just be vague about the details. If they ask for orders, just tell them they are being processed and delayed....In the end if you have a good gig, don't risk it.