maven-soapui-plugin has a parameter, projectFile which configures plugin with a path to the soapUI project file. Currently (in v3.0 of the plugin) it seems that referencing class path resources via projectFile is not possible. This would come handy if one wants to execute the same soapUI project/tests against different web services project modules (all of them sharing WS operation subset). A module could be created to hold soapUI project as "main/resources". Then each web service module could define same execution of soapui-maven-plugin by adding a dependency (shared soapUI project/tests module) to the plugin, and referencing projectFile from the dependency in same way as if it was in the web service project resources.
As example of an FOSS maven plugin where similar functionality works, see maven checkstyle plugin and it's recommended configuration on a multi-module project ( http://maven.apache.org/plugins/maven-checkstyle-plugin/examples/multi-module-config.html ).