Denti,
Yes, the FMC position must be a far superior navigation aid than any single beacon (assuming RNP is correct and ANP is within limits!). Which is probably why the Normal Amplified Procedures don't involve tuning the navaid for the transition from IAF to FAF/FAP and the FCTM declares that approach raw data monitoring is not required, except for an ILS or a localiser approach.
But as I mentioned before, Boeing's mention of "overlay" approaches is something else entirely and not generally what is meant by when other posters here are using the term "overlay".