I shouldn't really post in here 'cause I don't know any facts, and the last time I hung out with any of Ozma was in Fall 2007, but if Weezer did approach Ozma for the Fall 2008 tour, and "Ozma" declined I would simply imagine it was too late and they were too tired.
I don't think it's any secret that they had a rough time on the Blue October/Yellow Card tour... I mean, they had to drive themselves around in their veggie van and pay the official tour's soundboard guy out of their own pockets, every night of a show because he didn't want any of Ozma's connections touching his equipment. I don't intend to divulge "secrets" that were discussed with me on a personal/friendly level, but I think that's fucked up enough to be worth mentioning, especially because Yellow Card and Blue October had their own tour buses and certainly didn't have to pay $100 a night to run their own sound--and even if they did, that was obviously an expense they could afford based on how well the tour sponsors and managers took care of them.
Ultimately, I believe Ozma lost money and energy on that tour, and I imagine the band was just very exhausted from that bullshit. I wouldn't blame them for turning down a Weezer tour one year later, as from what I gathered, the band members needed a break. Not to mention Star had to worry about dressing Shiny Toy Guns.