I've found the Workgroup Manager to be tremendously successful in customizing the dock for the guest account. One thing I've found is that things will work better if all your account settings for Guest are made entirely in Workgroup Manager. That is, if you've customized the dock in Workgroup Manager, any parental controls you set up should also be in Workgroup Manager and not from System Preferences.

Before doing that, I had the guest account set in System Preferences to use Simple Finder and was frustrated when the dock didn't respond to any of the changes I've made in Workgroup Manager. However, when I deleted all the parental controls for Guest in System Preferences and re-applied them in Workgroup Manager, I was able to use both Simple Finder and a customized dock.

One other thing that I ran into was that both Camino and Firefox run erratically in the Guest environment and I'm not sure why. Safari works fine, however.