I have been running into the same problem over and over when I use the butterfly guard with a single underhook against good grapplers: They grab my pants and try to run around my legs. I'm getting better at defending those types of passes but I have to let go of my underhook to do that.
I'm especially annoyed by him grabbing my pants on the same side as my underhook, because he can go over my underhook arm and grab my pants on that side without me being able to stop it.
But very recently I came up with the idea that perhaps if I took a pant grip with my non-underhooking arm, I would be able to stop the pass AND do the basic butterfly sweep AND get to my knees to tackle him down (this last move being perhaps the most important attack).
Do you guys find this to be true?
(To explain better: I have an underhook with my right arm, grabbing his belt. Then I grab my opponent's pants near his knee on his right leg with my left hand.)