If a website is being promoted by lets say five members of it's community by posting on other related forums with a signature link. Can this harm SERP's, etc? These members are not being paid. They would be doing it as they have been members of the community for a long time and they wish to help by advertising.
Even five people probably couldn't post fast enough to get the "too many backlinks too fast" penalty. That's the kind of penalty that comes from having bots and software do the work for you.
It may and it may not. If it's too fast maybe like 100 posts in a day and then going on continously, it could be misinterpreted by the search engines as spam. However, if the posts are keyword rich and related to your site, I don't see there's any harm to it. I would work on the moderate scale to prevent all the troubles.
This scheme sounds more ineteresting. Think of the number of backlinks you will get for the similar keyword from the same forum. Certainly increase your site ranking but at the same time may harm the forum for being a link spammer.
Just make sure the forum is do follow and if they have the option to input your RSS Feed in your User CP settings, do so! Here's a list of do follow webmaster related forums http://www.linkbuildr.com/do-follow-webmasterseodomainaffiliate-marketing-forums/