Had to resort to this over the weekend after forgetting my belt and my pants kept riding down. To prevent the slack from forming "beak" in front of my crotch, I ran the guyline through the two belt loops on my side twice, doubling it up in that section. When I pulled that section snug it scrunched up evenly and stayed snug throughout for the rest of my 8hr hike. Didn't cut of circulation or slide down or anything. Saved about an ounce and a half over my nylon belt. Not sure if it makes much difference in such an easy to carry location, but it's an ounce and a half less on my back if I decide to hike in my underwear on hot days.
All the hipsters in town have been doing it for years, only with neon colored shoe string...
Best of all, it's the perfect length to make a fire-staring bow with at the end of the day! ;)