Well technically they can, but they're unlikely to. No, per NFL rules they have to allow him to interview for the HC position. Huh...sure enough. I don't understand why they bother asking permission then. Professional courtesy. Plus I think they have to specifically ask for them to interview for the head coaching position. Teams and coaches probably want their assistants to get interviews elsewhere because it gives the impression, at least, that the staff is well respected around the league.