How would IrrLicht handle cross platform post processing when it gets implemented? I know little about post processing, but what I read both DirectX and OpenGL support assembly input.
An interpreter could compile your custom script to ASM and pipe it into the current API. You could make the interpreter object a IrrLicht node so it could be read by .irr files; if it where to be a child of a mesh only that mesh would be rendered with post processing effects. I'm not too sure if that would be possible though. Reading it from XML would make it easy for render specific targets (ie, pixel shader 2.1 etc).
Because OpenGL is open source it may be possible to use its interpreter and use that assembly for direct X 8 & 9. It would be a huge benefit as there's a lot of OpenGL shaders out there already. Of course some custom behavior would have to be added, as the direct X 8 & 9 have limits on some shader features.
I remember asking someone who was working on a post processing class before; but I think he's going to make a simple class with predefined functions (ie; glow();, blur(); etc).