That's it. At the bottom (not top!) of the email that you get when someone sends you a message, add the following text:

To reply to this message, please visit http://bitbucket.org/user/whatever/inbox/message/id

It needs to be at the bottom because you decide whether you want to reply to a message after, not before reading it. At this point you are at the bottom of the email and so this is where the link should be.