They could have even included Sebastian as a familiar like in the original comics, and blamed the cat for their meteoric rise in fame instead of
subliminals inserted in songs

I was rather surprised there was no mention of Sebastian in the film.

Yes they should have had Sebastian in this.

They easily could have included Sebastian. Quote from the movie from right here at IMDB:
http://www.imdb.com/board/10236348/quotes
Alexander Cabot: You know what? I still don't understand why you're here.
Alexandra Cabot: I'm here because I was in the comic book
And I would have liked if they'd have included some of Sebastian's magical powers that were never included in the TV shows

I agree it was kind of dumb to leave him out. The whole premise of the band is cats, so they could've used him as a mascot or even just a cameo as a CD cover art. But he got completely neglected, it's weird. He was kind of a main character.

Exactly how would a cat do much other than walk around in a live action movie? I would love to see a cat help Val and Mel out of the TRL situation.