As far as Seagal goes, he was once a "cultural shihan" in Japan due to his rank and status as an instrucor. I don't think he ever received any kind of certification. As far as I know he no longer has an active dojo nor is he part of any aikido organisation (recognised by the Aikikai or not). Some of his former students are still active in the Aikikai. Larry Reynosa's organisation, the Makoto Aikido Kyokai, is a recognised Aikikai organisation.
Don't forget Matsuoka Sensei.