Lauren,
Certainly, you have a good 'feel' for who might be passionate about alumni and be able to provide the leadership contribution you're looking for. And truthfully, people like to be asked. If you approach some of these people you're considering and let them know that you believe they have what you're looking for, they will probably be willing to participate. You can open it up to staff/faculty to see who may have an interest that you didn't even consider. I would certainly invite some of your alumni to participate. Only THEY can really give you the graduate perspective. It sounds like you are well on your way to an advisory board!
June Gudeman