Hi Felix,
I saw that your post was a few days old and thought I'd reply. I have tried many times to use the Altiverb positioning and could never get it to approximate the kind of sound I was going for. I finally bit the bullet some time ago and bought the Waves S1 Stereo Imager. I now use Altiverb as a room only on my wet bus and keep its default positioning. The S1 Imager sits as plug-in on each track and determines the width and positioning left to right, while the wet/dry mix determines the positioning front to back of each track. I believe this is a far more realistic approximation of a real orchestra setting than using Altiverb positioning.
So basically my Altiverb instances (the wet bus) are receiving a dry signal that is stereo image reduced and rotated appropriately while at the same time my dry bus is getting that same stereo image reduced and rotated signal. At this point all you have to do is determine the right wet/dry mix to get the instrument as far forward or back as needed.
Cheers,