This feature is implemented in today's release in a different way than Emerald's however, since the latter doesn't allow real time adjustments (it requires editing your appearance to send the changes to the sim server), which pretty much defeats its very advertised purpose, and allows changing the X and Y bounding box size (which makes strictly no sense and could be dangerously abused for griefing purpose: imagine someone with a 10m wide bounding box on their Av and walking around in a crowd, for example...).
The Cool VL Viewer therefore implements Z offset (actually, bounding box size) adjustments in real time (each change is propagated immediately to the server), and provides an easy/fast way to change it via a new spinner in the movement controls floater. Note also that this setting is volatile (it is not preserved over sessions, unlike in Emerald), since it is only relevant to a given animation and makes no sense whatsoever to keep after the animation is stopped...
Enjoy !