First of all thanks Andreas for such nice plugin.
After some hours of trying I made working using Script Runner for getting a metadata ("mykey") from given project ("RISK")
Just want to share it with you this short snippet to save your time, I hope it helps 
Regards,
Jarda
import com.atlassian.jira.ComponentManager
import com.atlassian.plugin.osgi.container.OsgiContainerManager
import com.atlassian.jira.project.Project
import com.atlassian.jira.component.ComponentAccessor
Project project = ComponentAccessor.getProjectManager().getProjectObjByKey("RISK");
OsgiContainerManager osgiManager = ComponentManager.getInstance().getComponentInstanceOfType(OsgiContainerManager.class)
String x = osgiManager.getServiceTracker("com.osoboo.jira.metadata.MetadataService").getService().getMetadataValue(project, "mykey")