RGBHV (VGA) to YPbPr Converter and Waveforms?
The negative sync tip is easily done using AC coupling using an active clamp circuit during the sync positive edge with a 1 shot to short the output during the back porch.  Although your is DC coupled and relies on RGB being at 0V to make Sync Tip with R ratios, it shud be ok. 

You can make the video either 0.7V or 1.0V with negative sync at -0.3V. I prefer 1V for video.