Check the wiki page for hints, maybe.
Or, shortly, you can make her use your mouth during a battle if you Seduce her to get erection. It will be dangerous.
There are less dangerous options when you have a second type of Ogre encounter where you see another traveller searching for her. But there will be many ways to fail and not get to see the scene, Size Queen perk gives you more options that will lead to seeing an artwork... but at what cost.