I'm not sure if I get you correctly, but you can always download the JAR and install it locally using Maven. Something like this:
mvn install:install-file -Dfile=dwr-3.0.0-rc3-20130514.180049-1.jar -DgroupId=org.directwebremoting -DartifactId=dwr -Dversion=3.0.0-rc3-SNAPSHOT -Dpackaging=jar
more on the Maven info here:
http://maven.apache.org/guides/mini/guide-3rd-party-jars-local.html
EDIT:
I managed to install it from source as well by do SVN checkout:
svn co https://svn.directwebremoting.org/dwr/trunk/
My first attempt to build it failed because I was using Maven 2.x. Seems like DWR is using Maven enforcer plugin to enforce certain Maven version. I didn't check which version is really required but by updating my Maven to Maven3 I managed to install it.