Sorry aerobat77 - there are two possible pitchlock systems, serving different purposes.
The one I described above is a safety device, protecting against destructive overspeed in case of a governor failure on a purely hydraulic prop such as the HSD or Aeroproducts design. It works when oil pressure is lost and the blades are threatening to go full fine (low) pitch. Makes no difference if the engine is one or two shafts, or a piston engine.
The pitch lock you are asking about is used (I believe) only on props on a single-shaft engine. Its function is to secure the blades to full low pitch during a normal ground shutdown; this makes it possible to crank the engine to a safe speed for the next start. Without it, the drag of the blades would make it difficult/impossible to restart.
And as you note, a free-turbine 2-shaft engine has no need for the latter system, since the core engine can always be started with or without the drag of the prop. (In fact, in a helicopter with a rotor brake, the core can be started with the power turbine locked tight.)