It seems like it would be easily resolvable with TXT records these days. Anyone could try, say, on www.google.com to set a cookie for all of google.com, and the browser can fetch TXT records on google.com to see what, if any subdomains, it wants to allow this privilege for. Google could return a list or a wildcard; co.uk wouldn't allow any.

In a world without advertising, there's no reason why google.com couldn't also allow *.youtube.com to set cookies for it, but of course that would cause a tremendous privacy freakout. Though in practice they can and do just send every login/logout through a 302 redirect roundtrip to take care of the cookies on youtube.com.