I saw Warren Moon at multiple games growing up in the early 90's. I've also seen Bush Sr. back when Clinton had just taken office. Can't forget Big George Foreman. Staying on boxing, Diaz and Juarez are probably Rox fans, not confirmed just guessing they are. I've seen Justin Timberlake and Jamie Fox sporting rockets, so they could be.
I remember matthew mcconaughey was on nba on tnt promoting a movie and was thanking kenny smith for those championship years and all. He's also from the south so I could believe he is a fan.
Pretty sure McConaughey is and oddly enough, (Not a massive celebrity) Rico Rodriguez from Modern Family. His Twitter looks like an everyday Rockets fan account.
Y'alls boy BILL COSBY wore a Rockets hat on some late night talk show host back in the 90s, maybe it was Letterman. Or Conan. His legacy is similar to T-Mac's, he rapes, but he saves.