I'm thinking about guys along the lines of Greg Scarpa and Willy Boy Johnson...Guys who were rats for years but never testified in court.

Well i think your thinking of top echlon informants for the fbi the colombos had another he was allie boys best friend the guys nickname is blue i think his brothers a congress man in bk he was telling since the 80tys you got mark rosetti in boston they said he was feeding info for over decades flemmi decades. Whitey im still on the fence i dont think he thought he was a rat he was buying info but he new flemmi was giving info so its a toss up. You read the nyc fbi files in the 6070tys there was alot of rats who secretly gave info to be free.

Tony Anastasio.

Frank "Bomp" Bompensiero from San Diego was a decade long rat in the LA Family before he was whacked. sonny mercurio in new england became an informant in the mid 80s i believe which led to the recording of a mob induction ceremony. Scarpa tops the list of long time informants. Rumoured to give info as early as the 50s, definitely in the early 60s and up to the early 90s.

A lot of the long term informants were actually only associates and were never made. Salvatore Constanza of San Jose gave a lot of information on San Francisco, San Jose, Los Angeles, Bonanno, Colombo, and Detroit families, and he never testified. Gregory Scarpa is up there as known members who were informants, but there are a lot more who are not known. FBI came in in the late 1950's but the FBN had many informants. The one I am trying to track down is an informant for the FBN in the Bay Area from 1950's to the 1980's. The guy was made in 1954 or 1955, was close to the Cerritos, Lanza's in the bay area, Frank Garofalo, Joe Bonanno, Natale Evola, Salvatore Profaci, Joe Profaci, Joe Magliocco, Eduardo Castiglia, Pat Eboli, Joe Lucchese, John Ormento, and Joe Biondo of New York City, Peter Maggadino of Buffalo, Joe Barbara of New York, and Carmine Galante of Montreal Canada. All my suspects are cleared due to events, Gregory Genovese was made in 1964, Gino Barbara did not come to the bay area until 1958, Vincent Cirelli was only an associate, and Ciro Gallo died in the 1970's.
Information is out there, it is just hard to pin it down.
I would not be surprised if a document comes out and indicates Carlo Gambino as an informant. Salvatore Lucania was an informant, which people seems to discredit, even though there is evidence in the 1920's and 1930's. I don't consider WW2 efforts as an informant do to Lucky being a Patriot to the states. 1950's in the French connection case has info coming in from where Lucky was staying at the time. So there are many possibilities and the famous ones are the ones with intriguing stories in their life.

Jimmy Frattiano was a informant from 1967 to 1981.

Also whitey bulger against patriarca family