1900 presents one dominant challenge, time. 54 years after the last land grants were given. There wouldn't be a lot of original deed holders alive by 1900. The ranchos were intended to be used for farming. Once split up that use might have been lost. This would in some cases invalidate the property thanks to efforts to take the land, or descendents would simply have no use for the land anymore.
Obviously you know beyond squatters and the issue of getting government recognition of property, Californios weren't a heavy currency society and so much of what wealth they had was in property. To adapt to the changing culture they would have had to sell land to get cash. Many did. This was far more likely where larger cities were developing. There would be immense pressure to sell as the value of the land was rising. There was also the issue of property taxes. If one owned a large parcel of land in a less populated area, even this steady pressure of having to continuously pay the state government year after year for the land probably contributed to sales.
The original Spanish vision for the land was to give it to the Native Americans that had come to work the missions, but the Mexican government instead gave the land to Californios. As it became apparent that there was substantial interest by America to take California, the local government began handing out numerous grants in hopes of gaining loyalty. Not only to Californios, but to all kinds of people who lived in California with Mexican citizenship. The vast majority of all land grants were given in the last few years of Mexican California. This rushed effort left many properties ill-defined or without substantial legal documentation and made it easy for the US government to reject the claims.
Every property I've looked at seems to have fallen to descendants or others decades before 1900, and where descendents got the land, few didn't sell their property before 1900. I have found instances where subsequent purchasers held onto the land until today in their families, but not the original families, and there today seems to exist only 2 original land grants in single private ownership, one of them has remained essentially undeveloped.
Salvador Vallejo, Mariano Vallejo's younger brother, had Rancho Napa and Salvador, he sold most of the land from both ranchos relatively early on. However he kept a personal area for his home which he willed to his son Ignacio upon his death. Ignacio lived on the land until at least 1919 when the house burned down.
Rancho Boca de Santa Monica was given in part to Francisco Marquez. While portions of it were sold through the years, much of it stayed in the family past 1900, and in fact a portion is still in Marquez family ownership today.
So there were some instances, probably more I haven't found. But not likely many. So many natural and artificial challenges were presented. I found this very informative searching for information. Amazed at the lengths people had to go through to keep any land, and the lengths others went to take it.
One interesting instance I found was for Rancho Suscol. Originally granted to Mariano Vallejo, he had sold it to his son-in-law John Frisbie. Frisbie then subdivided it and sold it to more people. Then the US government declared the original claim invalid. This opened the land to the Homestead Act allowing people to move in and claim land for occupying it. Frisbie didn't give up however, and appealed to an older law that allowed previous owners to buy the land from the government before homesteaders could claim it. This failed too. But he had enough support to get a new law passed just for this one property, The Suscol Act which allowed him to buy the land and maintain ownership. However by this time the attempted homesteaders were beginning to make the same claims he did originally, claiming to be the lawful original owners citing the same act he did. It ended up being a US Supreme Court case, which ruled in favor of Frisbie and the homesteaders were evicted. So it took an act of Congress AND a US Supreme Court ruling just to save one Rancho in California that was being subdivided anyway.