I just got around to installing both of those plugins. I hacked up their provided init script to work with Debian standards. You can see the issue on github: https://github.com/MeetMe/newrelic-plugin-agent/issues/7
As for the varnish plugin I had to edit the gemfile to have
gem "newrelic_plugin", :git => "git://github.com/newrelic-platform/newrelic_plugin.git", :tag => 'release/0.2.11'
It would be fantastic if it eventually got as simple as their PHP plugin or regular monitoring is to install.