Would Spock's Beard be a good fit for a 90 minute headliner? Octane is a fine release, and they would pull in a lot of people. It's been a while since PP has had a prog rock band, and these guys lead the way in the US scene.
If it was Spock's Beard with Neal Morse still in the band, I'd say HELL YES, as they were one of the best live bands I've ever seen. Without Neal, they are just not as interesting.
I'm not sure a "pure" prog act is appropriate for PP. I think someone that is more guitar-oriented (like Enchant) would be a better fit...
I'd certainly be in there watching -- their performance at Dragon*Con (!) a few years ago is still one of my favorite shows ever -- but I've a feeling they wouldn't 'fly' with most of the audience, or more importantly, Glenn.