I'm doing something similar with DVD Studio Pro and a program called DVD2oneX. I make a 8.54 GB DVD in DVDSP, then build the DVD, but not write it.
I then use DVD2oneX to compress it to fit on a 4.7 GB DVD-R disc, then write the DVD-R using Toast.
Going this route, I've been able to squeeze about 2.5 hours onto a 4.7 GB DVD-R, with minimal quality loss. You should experience even less quality loss, since you're only compressing 100 minutes or so down to 90 minutes worth of data. The big caveat is that you can't use CSS encryption with DVD2oneX.