Here's a link to the animation we produced - pretty poor quality I'm afraid due to the incompetance of the web designers!http://www.pembrokebridgingcenturies.org/news-and-views/3d-animation.php
The animated trees worked 'ok', but I think they could be much better. We learned a lot. So far, I have found no other way to have animated trees in a scene. One of the main problems we faced (and still do) with Onyx wind is getting it to look strong enough to see, yet not so strong that it looks like a storm!
Also, the resultant vrayproxy files are HUGE!
In terms of wind direction, obviously you can't rotate all the trees randomly otherwise the wind will appear to be coming from everywhere all at once. Probably best to random rotate over a 20 or 30 degree limit I guess.
Good luck. Trees are hard. Animated trees are harder!