It's on a per-site basis I'm afraid.
Thank you very much indeed chkdgate

but actually the most helpful thing someone can do for me is leave the link there. It helps with my website's PageRank and very occasionally does bring in a customer or two. A small donation for branding-free doesn't cover the benefits of having a footer link present, I just threw in the option as a curteousy for those who were really bothered by having the copyright displayed.
But it's all good, I appreciate it either way.