For your information http://trac.openscenegraph.org is the old OSG website. So you are looking for wrong information. The last dev version is 3.1.8 and even 3.1.9 has been already tagged in trunk : http://www.openscenegraph.org/index.php/download-section/developer-releases
OSG seems to bring some big changes for now... I think we shouldn't add the patch about "Ensure compatibility with OSG
3.1.8" for the 2.12.0 release and simply announce that FG 2.12.0 require last stable OSG 3.0.1 Once 2.12.0 is released we could spend some time to look at updating our code in compliance with last OSG.