Or better yet, place an unconcealed, non-functional camera pointing up through the floor. Giggle maniacally whenever someone has to go.
Or even better still, place an unconcealed, functional camera pointing up through the floor. Offer $20 if they're "willing to put on a show". Get them to sign a waiver, allowing you to post the video on the internet. Profit.
Just tell people you're poor, and just trying to pull a Zack and Miri to get out of your slummish conditions. When they say they aren't willing to cooperate, point out that you have to live in an apartment that has a hole in the floor of your bathroom, where anyone can spy on you taking a whiz.
Sell it. Turn those lemons into cold, hard, degrading cash.