One of the usual solutions for this is to install a second follow focus device, but attached to the zoom ring on the lens. It can get pretty crowded if the follow focus devices are robust, so mounting one on the opposite side helps, but does not make hand holding and operating things very easy....unless you have several extra hands to help ;-)
For most movie work, the zoom effect tends to look more like old Super 8 camera work, but there are times when zooming in while shooting is the only way to get that effect. Most video is captured at one focal length, then zoomed, then captured, then zoomed, etc., and the clips are put together in ways where appropriate transitions help the overall effect. But it you want a zoom in the middle of the clip, and do not want things to jump around too much, placing another follow focus device is about the only way to go.
LJ
P.S. Check this option out:
http://www.idcphotovideo.com/blog You can mount two of them, or you can mount one different type of unit for the follow focus and one of these "friction type" units for the zoom ring on most lenses.
And here is a Vimeo video showing two units attached to facilitate focus and zoom with the 70-200 lens:
http://vimeo.com/11904897