I was going to suggest the camera idea, but now I'll suggest getting equally comfortable around them. Shower with the door open. hang out in the living room in just a towel. buy a speedo and flaunt it.
go to goodwill and buy assorted sizes and styles of used panties (they do sell used panties right?) randomly hide them under his pillow, in the couch, in his car, etc. enjoy the show
Dude, you're entire idea is gold! 
I did the "equally comfortable" with my old roommate, minus the speedo part. Worked perfectly.
Yeah, it's all fun and games until they ask you to join them in the shower.
Never happened. He did, however, tell me that it made her, his gf, uncomfortable and asked if I would stop. I

and told him I'd stop when they did. She stopped staying over.