Looking at the source code, it doesn't look like there is any support for proxy servers when downloading a plugin. (/usr/local/www/freenasUI/plugins/plugins.py just seems to have code to go connect right to the hardcoded URL.)
You might want to make native proxy support a feature request. It may already be one. I didn't look in the bug tool to see.
Alternatively, if you have a way to put a transparent proxy in front of FreeNAS, you could work around the problem that way.
Or, you could download the plugin via a web browser and use the manual upload option to load it into your FreeNAS box.