While killing time the other night waiting for the hour to change in a search for Colosseum tickets, I came across a listing for P.Stop, associated with the Roma pass.
https://pstop.it/ [may require browser translation]
They appear to be a series of tourist information kiosks scattered across the city - all offer paid WCs - which may be underground beneath the very modern kiosks at street level. Some offer additional services, such as a ticket agency. Just wondering whether anyone has come across one.