Now you can see more on 2d filters.
i complete the 2d filter, and now there is filter named, custom filter that you can write your own GLSL for 2d filtering.
see http://blender.gameengine.googlepages.com for more information.
source code is available via game_engine branch https://svn.blender.org/svnroot/bf-blender/branches/game_engine/
EDIT: now you can download the latest windows build of game engine branch in http://blender.gameengine.googlepages.com/downloads