I found this with google and tried it, and I have to say I really like it, because it works. But on my Intel card it is slow, but that video card sucks anyway. I think the performance could be better if you exploit the fact that GUI elements update rather seldom compared to the 3D Scene. So instead of moving the 3D Scene into an ImageView, you might render all GUI elements that have changed into an offscreen texture and draw thaw with LWJGL manualy. This might be done, if you can replace or modyfy the scene Renderer from JavaFX with one of your own.
I tried to do something similar with swing, but I miserably failed. I could copy the swing component to an OpenGL texture, that was no problem, but I was not able to do it on change events. Also all interacitivity was lost if I wanted to bring swing elements into the 3D space. I was not able to find a hook to replace the renderer or to replace the user input. If you can do this in javafx, I will like you very much