I haven't really done many rainbow rails like that, but my instinct is to use the front truck as the main guide for the rest of the move. Not necessarily being heavy on the front, but "feather" it for the rest of the trick to follow through. More of a glide than a grind. Maybe be aggressive initially and back off once you're in the grind?
Seems like a trick that takes strong commitment and speed, but even that has a strong chance of bucking you (as you saw first hand, ha!).
In the Ben K clip, it looks like he skips a lil pop to get the angle to make the rainbow. Sounds deadly.