Access root domain's localStorage from subdomain

EDIT:

For the sake of simplicity:

I've got a simple UserJS script (FF/Chrome: Greasemonkey, Opera: built-in) that fixes some issues on website I'm using quite often. The problem is that I need to store some data between different requests. My first attempt was LocalStorage but it fail when it came to work with subdomains: www.domain.com and subdomain.domain.com (unfortunately root-domain is subdomain in fact - stupid www) . I need to be able to access some data source that would be available everywhere.

Now I'm stuck - any ideas?