MarbleWidget and MarbleMap provide their full amount of methods as a D-BUS Interface. Therefore you can easily create a shell-script to control Marble using the qdbus command line tool.
Before you start you need to find out the PID of the Marble process: In your script you can do it like this.
Then you can pass this PID to the actual qdbus call
qdbus org.kde.marble-$marble_pid /MarbleWidget
This call returns a list of all the methods that MarbleWidget supports. From this list you can choose the one that you'd like to call, e.g.
qdbus org.kde.marble-$marble_pid /MarbleWidget org.kde.MarbleWidget.zoomIn
to zoom into the map. Note that for function calls you shouldn't pass the brackets "()".
As a convenient tool you can use qdbusviewer to get a graphical list of the interfaces and methods that get provided through D-BUS.
Problem: Neither the MarbleMap nor the MarbleWidget interfaces are available in my installation (Debian lenny), and qdbus complains:
org.freedesktop.DBus.Error.UnknownObject (No such object path '/MarbleWidget')