Travis,

Go to the California Gardening Forum and page down in the list to find a thread I started called "Graft and Corruption". There you will become aquainted with a fellow named Joe Real, who has done a lot of grafting of fruit trees and is very knowledgeable.

You may in fact find your answers in that thread and if not, perhaps Joe can help you out. He is more involved with fruit trees however and not hydrangeas.

Note that the thread reveals that grafts do not typically hold across plant family lines, if at all or rarely so.

Good Luck