The best historical drama I've seen with a girl disguised as a boy in the military is Oh My General (2017) . The love story is so cute. This girl goes all the way in her act, living the life of a general, with several concubines. Also there's another girl who's deeply in love with her. Beautiful soundtrack as well. The English sub version is still on YouTube.
A bit like Empress Ki but much lighter is My Bratty Princess (2005). This Chinese series is about a girl dressed like a boy traveling with a street gang. She meets the emperor in disguise. Also she doesn't know she's a princess herself. Starring Korean actress Jang Na Ra. Still available with English subs on Daily Motion and Drama Cool.
Furthermore MyDramaList uses hashtags that present drama titles like cross dressing. Or try these custom lists: Gender-Benders: Girls in Disguise and Cross Dressing in Asian drama and movies. Enjoy!