This is probably a noob question, so forgive me. But couldn't you join a super popular, highly ranked forum, and make a few regular posts in a big thread or something. Once you do that, go back a little while later and edit one of those posts, and stick your backlinks there. Seems like the chances of it being flagged are greatly reduced, since the post should be in a big thread that should now have numerous posts after it so people won't really pay attention to yours. Not like mods/users actively go through all old posts.