If you give someone a swirly, you risk exposing them to harmful bacteria from the toilet water. Is there any way to clean a toilet super thoroughly beforehand to make it safe?
If not, if someone hypothetically installed a toilet in their basement that no one used for its intended purpose, would that be safe to use for swirlies? Particularly if you let it feed from an isolated water tank instead of the house plumbing and drain into another tank.
If the toilet isn’t used, the water should be pretty clean, should just be using normal house water. You probably wouldn’t want it to sit for too long, and it should be cleaned occasionally with something that will kill bacteria. If it’s used infrequently, I’d turn the toilet off at the wall, then flush the toilet a few times to bleed out any standing water.
Also, if it’s a kink thing, be sure to have some sort of safe gesture or dead man’s switch or something. Or a snorkel, lol.