So I decided to skip the event with Atusa in the north entrance of the Fort Joy area by climbing the ladder to the left (to the area where Yarrow is stationed). After proceeding through the story and slaying Orivand I remembered the fact that he held Dallis' panties on his person the whole time. So I went back to Dallis and Atusa in hopes of there being a special conversation option where I present her panties to her and tell her who had it all this time.
Turns out, there wasn't such a conversation.
So here's my suggestion: have Dallis react to you when she notices that you're in possession of her panties (whether it be anger due to her having searched for it for days and seeing it in your hands, or her actually grateful that you've found it) and wondering where you got the pair from (this is where you can confess that Orivand had it, or you could lie and tell her that you've had it with you since you got here, or found it on the beach), giving you an item or gold in return for it.
This would make for at least an interesting mini-event inside an event.
