I think we can dispense with the medicus's certificate and accept transvestites. (After all, sex change operations would be anachronistic in 400 AD, Elgolabalus (sp?) not withstanding.)

In any event, anybody who wants to write the story of a female warrior who disguises herself as a man to enter the legions is welcome to do so. If, however, she's discovered in the fraud, no doubt there will be unpleasnt consequences.

Legionaries still bathed naked didn't they?