HiRez allow this 3rd party workaround to be used (at least for now), but I can't publicize it. The purpose of this script was to allow you to simply hold down left-click and continuously fire without needing to spam click. It was primarily written for the HeadHunter and Tremor launcher, but works for any weapon that doesn't already have Hold-to-Fire. However, due to limitations of the script language, it actually slows down the fire rate of automatic weapons with a High RoF (like the SMGs, IC, MG, etc). It may also interrupt Beam Healing or Robotics Repair.
I'm not sure I'm allowed to share this script with people who request it, so if you're interested, PM HiRezDuke, and if you receive approval, I can give you the guide/script.
If you would like this integrated in game, please show your support here.
Who knows, if enough people ask, maybe they'll add it by default? It's already there for the Magmalance. Why shouldn't it be on the other launcher's at least?