Thanks to both of you for your great advice and explanations of the slow sync. I was under the impression that slow sync would act as a "fill" flash. I didn't realize that the shutter speed would slow.
We did the shoot today and just stayed with TTL-BL and it worked out fine. We used the pop up flash (didn't want to have to put on the SB800) and it worked perfectly. We were photographing a subject wearing a hat during the middle of the day with a high sun and just needed to take the shadow from the hat off his face.