This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
You can manually download and unpack a Maven plugin just like other dependencies inside $HOME/.m2/... Just make sure to use the usual directory structure and to specify the plugin version explicitly inside your POM.
But why don't you simply configure Maven to use your proxy? Maven should work behind a HTTP proxy without problems.