I personally like the idea but not all people do. My mother, for instance, thinks it's a huge cop out. She donates a lot to charity and doesn't need anyone making her philanthropic decisions for her. And yes, she's impossible to shop for. Unfortunately this acorn doesn't fall from the tree
If she hasn't been an active member through the years, there may be some member recognitions she hasn't been given. For instance, Alpha Xi Delta has an Order of the Rose pin for 50 year members. If your sorority has something like that and she doesn't have hers, maybe you could arrange for her to get it.