I have just subscribed to newsletter by using this link: http://stackexchange.com/newsletters
But you need to use filter first and I didn't found way to pass filter trough query string.
I unsubscribed and looked around SPSE for link. I think I saw it once before but now I cannot find it.
I think this is preview of current newsletter: http://stackexchange.com/newsletters/newsletter?site=sharepoint.stackexchange.com