while it is true that Hells Canyon is dam controlled, it is also possible to get big water at that time of year. 2012 was a decent water year, while the peak that year was before your launch date it was well above the control limits of the dam:
2011 was a big year, with peak right around your launch date:
one option for shuttle that I have heard of recently is to jet boat back up. two friends that have done it said it was about the same cost as the shuttle service, and was really fun to see the canyon backwards.