I think they can basically keep it all under wraps.
a few points:
1. with Fewell, officially interviewing him was something the team had promised him, and it was also a way to help him raise his profile in the league. It also fulfilled the Rooney rule, but that ended being moot anyway with Frazier.
2. With Frazier, I think it had to be an "official" interview because his team is still alive and league rules dictate a certain procedure in these circumstances.
also keep in mind that the Redskins didn't have any kind of official interview with Shanahan until it was basically a foregone conclusion that he was the one being hired