Checking DOFMaster, after a few trials, I decided to set the 7D for manual focus at the hyperfocal distance of the Canon 15-85mm zoom at 35mm focal length. With an aperture of f11, the hyperfocal distance at 35mm is 18.8 so everything from 9.4 feet to infinity should be in focus. I guessed that the 580EX II would have enough power at f11 even shooting through the Lumiquest softbox because it was close to me (it did). I set the flash ratio at 4:1 and let the Canon ETTL system control the flash. The Yongnuo 460-II was set at it lowest flash power setting.
The 7D would be shooting at f11 for depth of field. I choose manual exposure and dialed in a shutter speed of 1 second. The combination of flash at ~ 1/1000 second and shutter at 1 second might seem odd but the flash exposure was for me and the longer exposure was to catch the fireworks.
After setting up the equipment, to my aggravation, the 7D did not trigger the 580EX II. Finally I realized that with the 7D in portrait position the on-camera flash was on the left hand side and the zoom lens was blocking the triggering flash from the view of the 580EX II which was on the right hand side. I then moved the 580EX II from the camera right hand side of the scene to the left hand side and also moved the background/hair light from the left to the right. Suddenly everything worked as planned.
Unfortunately I missed most of the fireworks activity because of having to set up the gear twice. The introductory picture was the only one captured with any significant amount of fireworks in the sky. Wait 'til next year!