Sarah Michelle Gellar dressed up like royalty for a Princess Rap Battle, and she learned there is a downside about wearing that type of gown. What happens when you have to tinkle? If you are Buffy, you find a way. It will be a messy one, but it is a way.
On a positive note, at least she doesn’t have to worry about an upskirt or showing more than she would like to while she is doing her duties.