Today I was successful modifying the guest information for a reservation on Marriott.com and adding my wife's Bonvoy number to the reservation. It now shows up online. This was an II reservation and I noticed that the phone number associated with it was the one from II and didn't match the one in her Bonvoy profile. I updated that at the same time and it stuck. The reservation now shows up under her trips on Marriott.com. Not sure if it was the phone number that made the difference or just how this reservation is 'setup' that made it possible. Now to try it on a bunch of others.
Edited to add: I just did it on another and didn't change the phone number to match and it worked. So based on past experience with other reservations, it seems there are some reservations that simply won't let you edit the guest information online.