There are these two big forums I go to and they have this "Comings and Goings" thread wherein members who are taking a break from the forum usually bec of work, exams, weddings,etc. post a general message and those who had been in hiatus for a few weeks to several months/years post that they're going to be active again.
Maybe somewhere near the Intro thread? I saw a post in Community (the rare time I strayed there) about a forumer who had been inactive and it was good to read the people who had known her to say hi and welcome her back.
Maybe it can be made clear in the thread description. I think it'd also be nice, forumers wont feel like nobody will notice they have disappeared. Or if they have disappeared, that they had been missed or sorely missed Breaks and re-enters