Yes, if you read the book version of Legally Blonde, Elle's a DG. And in Eating the Chesire Cat, Tri Delta is prominently mentioned.
So it doesn't seem like there's any restriction on using GLO names even in less-than-positive portrayals. However, I think most writers don't want to risk the fact that they could possibly be sued for libel if they portray real sororities or fraternities in bad ways, so they make up their own names.