Thanks for the reply! Didn't notice the Detach Selection yet, I can use it together with Forest Instanciate to turn it into an animatable object.
Is it possible to add a button 'Detach as geometry' to do this in one step? Ie, the selected Forest Trees will be replaced by the original (custom) geometry with the current position/rotation/scale and are not part of a Forest object anymore?
Or can this be done with a script?