Thanks, that works well! I can see from my proxy logs that it's now doing more work, and I can now do tekkit things. I'm on linux, so the resulting command looks like this:
mkdir .techniclauncher/rtemp
java -Dhttp.proxyHost=localhost -Dhttp.proxyPort=3128 -jar ~/Downloads/technic-launcher-latest.jar
It's not documented anywhere though, either that or I missed it. It does look like a verbose debug session.