I'm not sure what is meant by "hard coded" ... the names maybe? Maybe I am misreading the question, but Mike's answers isn't very helpful.
You CAN modify the projectile as long as you keep the name of an existing projectile.
As long as you don't intend to use "rpggrenade" and your custom "plasma gun" then just modify the rpg into your custom weapon(without changing any names).
I would love to see TGC release complete info on weapon customization, including ALL possible settings in gunspec.txt. I would like to see a complete explanation of requirements for \entitybank\weapons, \gamecore\ammo, gamecore\guns, and gamecore\projectiletypes.
The lack of proper documentation is really annoying to me.