Hey guys, I've made considerable progress with using proxies with the technic/tekkit launcher. This is the command I use in a .bat file with SOCKS, HTTP, HTTPS proxy servers. Hope this helps!
java -Dsocks.proxyHost=65.175.251.250 -Dsocks.proxyPort=1757 -Dhttps.proxyHost=23.21.217.44 -Dhttps.proxyPort=3128 -dhttp.proxyHost=50.57.175.200 -dhttp.proxyPort=80 -jar technic-launcher-latest.jar
I've tried running the jar in command prompt but it reboots with different parameters. The exe isn't considered a java program, so it doesn't run with proxy.
This command works perfectly with the minecraft.exe
Hope this helps!