This just refused to work because the flashplugin-installer that actually downloads the packet doesn't honor the http_proxy environment variable to I had to do a little hack to get this going. It's actually pretty simple: download the file and serve it from localhost so we don't need the proxy at all.
- mkdir /tmp/flash
- cd /tmp/flash
- wget http://archive.canonical.com/pool/partner/a/adobe-flashplugin/adobe-flashplugin_126.96.36.1990.orig.tar.gz
==> or whatever version YOU need
- python -m SimpleHTTPServer 80
- unset http_proxy
- vi /etc/hosts
==> add an entry so that "archive.canonical.com" points at 127.0.0.1 and save the file
- aptitude install flashplugin-installer
- vi /etc/hosts and remove the newly added entry
Stop the python http server and you're done.