It is too late. There is absolutely no way that I would take all of this work apart just to add cornerstones. Cornerstones does give some exitement to the eye but with the work you have already put into this quilt, I don't see the need to undo what already looks good.
If you really want them and have to have them in this quilt, you can add cornerstones on the first border and then add your last border. Or you can start a new quilt and add them.