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.