in fact! ... somehow I had the idea to rig xFrog trees and fortunately I got a not too bad result quite fast (the finals are a bit better than the blender - test renders I showed here.
Critical are the beginning and end points of the loop. It's not so easy to swing 10-30 branches and to synchronize at one point, but it should absolutely not look in sync. The rest is a bit intuition: amplitude and frequency depend on thickness of the branch and its position in the tree. Some "dirt" in the frequency by not using pure sinewave but changing it manually helps to get the look of some wind gusts. All this for 250 frames and then you have a loop that's at least as good as the Unity or CryEngine blending function of plants.
For a better overall result it makes sense to copy the loop of 250 objects and move the first 20-100 obj/mtl files to the end by renaming. Then you have another instance moving not unnatural synchronous. If you repeat your loops 2-4 or even 5 times it still looks natural.