Jeff Genender writes about propagating dependency versions in the POM to sub-modules. He uses XML entity references.
In our current project we’re using a build mechanism of several modules, but propagate settings and POM info through the projext.xml extend element. We have one master project.xml file, that all other modules extend. If one of the other modules has other dependencies/info you can override/add it in the local project.xml file. This should propagate both POM and normal properties.