This is inspired by the gargoyle that tests your blood for aberrations in the Lowerdark.
Maybe have something similar outside of Sanctuary's gates that not only checks your blood but takes your ring when you're leaving. When you come back, it checks for aberrations. If you're fine, it gives your ring back.
The hooded man in the Illusion already handles this, doesn't he? The text of the Quickfeet Memorial Act suggests as much, anyway.
Or players could use initiative and use the government system already in place to create such things.
This is already in game. Hooded man in the illusion takes and keep your ring and gives it back after aberrant tests.