I just stumbled across Irrlicht, while looking for prototyping engines..
My question is how tricky would it be for Irrlicht engine to support/load the open Flight scene format (www.multigen.com).
I know it is a long shot, but it would allow for some pretty big enviroments with all the funky flags that openflight builds into its model database..
Open Flight scene graph
I think it would not be a big problem to add this functionality, the engine is quite extensible. You simply will have to implement your own Mesh Loader for this. And if the culling algorithms already in the engine won't fit your needs, just add your own scene node to do this. There is also a tutorial about this on the website.