Use a text editor to edit the cybrid or human campaign file in the Starsiege game folder. Find the "usePlanetInv = true;" entry and change it to "usePlanetInv = bool;". Now find the "techLevel = 3;" entry and change it to "techLevel = 10;". All Hercs and unlimited weapons will now be available during the game.
Note: Whenever editing a game file, you want to make a backup just in case something goes wrong.