Cool. Seems like there should be some security to prevent abuse though, otherwise surely I could just add a ton of subscriptions to pages on a target site, and

Cool. Seems like there should be some security to prevent abuse though, otherwise surely I could just add a ton of subscriptions to pages on a target site, and start getting your server to DoS them? Perhaps have it look for an 'allow' file on the destination first, and if it's not there, don't send any further messages?