The graphics rendering system is the core of a 3d engine and it has been deeply integrated with the engine.
So it's not so easy to extract it and replace with another gfx system.
The graphics system of CS is powerful enough.
Not like the case of network middleware. One implementation of CS is based on HawkNL but it can also be replaced with other netlibs, such as Raknet, I think.
Well, not sure about irrlicht, but for OGRE, I think it should be use as a graphic engine, since that's what OGRE is.
Will a partical creator be in CS like the one that's in OGRE ?