Use SFML 2.0 and render to texture. Get the texture id (this will require hacking SFML a tiny amount). Render your 3d scene in irrlict, using the 2d texture on a plane. Profit.
Wouldn't that require to separate rendering contexts? And, uh... due to the way those two libs are built, two windows...?
I presume it would be good to just use pure OpenGL with SFML but... I know nothing of how to do it either.
It's simple, just invoke OpenGL functions
Ok, it's simple for 2D - I've done it in a 2D game to do stuff that SFML couldn't do - cool lightning rendering, clipping in GUI, a circular health bar, batch rendering, normal mapping... etc. To use 3D with SFML you need to set up the OpenGL state for 3D rendering, then change it back to 2D for SFML code. To use SFML in an Irrlicht game you'd need to the same thing in the other direction
Setting and restoring the states shouldn't be much trouble - you should look at pushing the Projection and Modelview (and possibly the other two) matrices to the stack, using glFrustrum/glOrtho to set up the viewport, enable/disable depthtest and that should more or less be it. Probably a combination of Pop/PushMatrix + some copy-paste from either library's initialisation code would do the trick.
How much SFML do you use? Text and sprites, or something more? I still think picking SFML apart and using the interesting bits in an Irrlicht game is the way to go... SFML is waaay simpler than Irrlicht.