Yes SpeedTree supports Amblic IO, it also supports FBX, both with wind animation embedded.
I am enormously impressed by the game engine output of Lumberyard, Unity, Cryengine and UE4 and this is not in a small part due to their support of fine motion animation in vegetation.
Grasses and leaves softly moving in the breeze can really sell a scene. As far as I know Speedtree is the only package that creates wind effects "out of the box" and so supporting this in Terragen would be a huge benefit.
I know Terragen can support obj sequences but this is cumbersome in comparison.