Share a common page between blogs?

Hi,
I’m trying to adapt a ‘subscribe to blog’ plugin to work with WordPress multisite. The plugin has a subscribe page. I need the subscribe page to work for every blog in my community of users, with reference to the current blog. Preferably showing with the current theme too. I’d rather avoid creating a subscribe page automatically for every blog, e.g. on its creation, because then I have to exclude it from the blog’s navigation menu too. Possible but messy.

I have a solution in a sub-directory install. I put the subscribe page in the root of the WP install. Then, when I link to it, with the current blog url included, it shows with the current theme, and references the current blog. For example, this link works – http://myblogcommunity.com/a_persons_blog/the_subscribe_page.php

This solution doesn’t work with a sub-domain install however. For example this link – http://a_persons_blog.myblogcommunity.com/the_subscribe_page.php doesn’t seem to find the page in the root of WordPress.

Any ideas how I could get this to work? I guess it’s to do with the rewrite rules. Thanks.