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.
There’s really no need to isolate a “dedicated” toilet from the rest of your plumbing system. If you don’t use it for bathroom business, a new, never used toilet isn’t much different than a sink.
The only issue might be the water in the tank getting stagnant and developing biofilm. The best way to prevent that would be to install a commercial-style toilet with a flushometer valve instead of a tank. Might also give you the “institutional” feel if that’s what you’re looking for.