Vue does this with a simple checkbox and a wind speed/direction parameter. The thing is, Terragen can pretty much do everything Vue can(render wise, I mean, we don't have plant generation or unique instancing yet), you just need to tell it how to do it.
It's a shame that the model geometry can't be bent, maybe by altering the world position of the bending noise but, as soon as it's added to an object, the noise takes on the 'object normal' instead of the world coords.
Still, I think it looks very nice, for something that isn't an actual included feature of TG. Multiple objects all blowing gently in the wind would look fabulous, I think.