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.
Flush the toilet, and then clean it with bleach. For added protection, have the swirlee wear goggles, maybe earplugs and a nose plug, and have them close their mouth.
If you’ve watched Jackass, Steve-O volunteered to sit inside a porta-potty as it was hurled into the air. If you don’t mind seeing shit fly everywhere, you can look at the PPE he was wearing. Your swirly will be like that stunt, but much cleaner.