ThomasM Hi Thomas - thanks for using Ziva!
There isn't a built in bay to paint the zRestShape influence it the moment.
The work around for this would be to do the following:
Use a seperate blendShape node per shape where the source is the neutral mesh, and the target is your "pulled back" shape.
You can then use either the deformer weight or the per shape weights to achieve what you are after - as the connection is live.