Even checking some cross-dressing websites, most cross-dressers would like to cross-dress in uniform, but can control themselves enough to be happy with just doing it in private, or they underdress. I cannot find a single issue to ever come up dealing with cross-dressing in the military. In fact, I can't even find a case of someone being discharged from the military for being a cross-dresser, when not in uniform. Of course, most cross-dressers are even more secretive about that side of their life than homosexuals, so that may account for it. Society, in general, looks down on cross-dressing especially M2F cross-dressing, even more than it does homosexuality. This is the main reason why people mistakenly believe that all or most cross-dressers must be gay. It is just widely misunderstood everywhere.
Wearing a military uniform is about professionalism, so even if there were cross-dressers that were allowed to cross-dress, they would still be required to wear the uniform of the opposite sex and obey the regulations dealing with wearing the uniform of the opposite sex.