Here are some ideas for what you can say when a person is trying to beat down the door while you are indisposed:
1. Hellooo. It's me- Please say this in Adele-esque way or the desired effect will be lost
2. Come on in, the water's fine- That way they will know someone is in the room they wish to use. With this phrase, they will also know the person using that room is not selfish.
3. Wait your turn!- I put the exclamation point on there, because you should probably yell this for best effect
4. Can I help you?- This shows the person on the other side of that door how compassionate you are. Not only will they be comforted by this knowledge, you can check helpfulness off your list for the day.
5. Don't come in here! The zombies have invaded through the roof!- Yelling useful tips to the person outside will also help them gather valuable information during their long wait outside.
6. I need another 10 minutes to make my hair look perfect!- Everyone understands what bathrooms are used for. Some people just need a quick reminder.
7. Who is it?- If they respond, at least you'll know who the impatient person on the other side is. This could be helpful for the next time the bathroom is locked.
9. One moment please- This is probably the most polite response. Sometimes they still rattle the handle after you speak to them; connotations imply an emergency.
10. Sorry! You can't come in! The door's locked.- This is the common knowledge answer. Now you're both on the same page. If you are at a gas station, you can also add "The key's in here with me!"
11. - Silence. This will leave them wondering. Be prepared for additional handle rattling and perhaps a surprise entrance from the janitor with the handle shaker peering over that sweet janitor's shoulder.
I'm sure you have some great ideas yourself now. Don't get too crazy, you never know who will be on the other side of that door.