From what I read here, you'd do something like:
mvn install (to make your shared and client artifacts available to the server one outside the reactor build)
cd *-server && mvn appengine:update
Of course, that's after you configured the appengine-maven-plugin in the *-server project's POM.
Otherwise, the tool seems to be the com.google.appengine.tools.admin.AppCfg class in the appengine-tools-api.jar from the AppEngine SDK, that you'd call with the "update" command and your WAR file (or exploded WAR directory, i.e. *-server/target/*-server-*/ after a "mvn package", I can't tell) as arguments.
(note: it took me 10 minutes to find that information from the Maven and Ant documentation pages and a bit of googling; I never used AppEngine)