After much code hacking and testing by the people of #marp , there is now an option on the bottom of each forum page to watch that forum - it will send an email to you when a new topic is posted (but won't send any more until you actually visit the page - good for reducing the amount of email sent out).
And you can still watch just certain topics, with the similar link on the bottom of the topic page.
Thanks a lot for the help, guys!