Something tells me Roxas didn't really have a whole lot of adventures with Organization XIII.
Roxas only existed for about a year before the beginning of KH2.
Riku started helping DiZ and Namine at the end of CoM, which means that Roxas only existed and worked with Org.XIII for about a day before Riku joined forces with DiZ and Namine.
It seems that the only major job DiZ and Namine gave Riku to do was to apprehend Roxas and bring him to Castle Oblivion. DiZ was already fully aware of the location of The World That Never Was, so Roxas wouldn't have been difficult to find.
The only factor that may have delayed Riku's mission was the fact that Roxas defeated him during their first fight, and that Riku had to reawaken the Heartless of Xehanort in order to complete his mission. Xehanort had been fighting for control of Riku the entire time, so reawakening him couldn't have taken very long at all.
Since apprehending Roxas was Riku's only mission, and it took place about a day after Roxas' creation, that means that Roxas was only with the Organization for about a day before respawning in DiZ's artificial Twilight Town.
I just dont think there's many valuable stories for a spin-off to tell that only happened during the course of a day.