I'm not sure it matters enough to have the ability to do exact placement, since the interface isn't quite as easy to see things (at least to me) without doing short test renders.

And if the actor is sitting in the grass, his/her feet can be imperfectly positioned, since they would actually be IN the grass. The trick is getting something to satisfyingly muffle the contact points so it looks natural. My biggest complaint with 3d pictures is the perfection....all lines are smooth and even on terrains, there is no real matter extending from them. Even with the new displacement materials, there are no blades of grass...and the grass plant isn't very good for tight shots anyway...it's very much intended for background (as are all of Vue's vegetation).

For things like that, I would put the actor and some important props together and import them together. Then, it's just a matter of positioning the little group into the scene (possibly to  replace a pre-existing chair or something).

Carrera's hair as grass, though....that has a lot of potential once they get the poser issues worked out.