i would be great if you can pass in the ISceneNode (the camera for example) , depending on the location of the Iscenenode to the tree, the LOD will change automatically.
Did you look at the source? Try and see the CLODSceneNodeAnimator. Right now it does exactly that, only that it only does it for the active camera. If you want it to use an arbitrary node it's a very easy fix, though.
Also , if the distance if far enough, i think we can still show the whole tree by simply showing a picture that rotates perpendicular with the character, as the character walk closer it would then switch to the tree you have.
My code is for generating the trees models. Making it change to billboards is easy, but it has nothing to do with trees. Rocks, houses, humans, castles, etc, are also worth turning into billboards at a fair distance. I could write it, but I don't see why you would expect it in a thread about trees. Afterall, this is not "ACME Forest Creation Kit™".
I really need to do something like this for my own game, i'll be willing to modify your code myself...
My code is completely independant of what you want to implement. The ILODMesh and ILODSceneNode might need a new virtual method or two depending on your approach, but you really shouldn't touch the tree generation itself.