What's the reason for locking the shoulder harness? Does your airline not have tension reels on the shoulder straps?
All the military types I've flown had inertia reels and the checklist called for "locked" during take off or landing. This was impressed upon us during ship board operations, that an arrested landing with an unlocked harness may see you making intimate acquaintance with the gun sight, and undergoing facial modification without the benefit of anaesthesia. Whether that ever happened or not I can't say, but seeing that the manual for the particular aircraft says 2 to 3 "g" was necessary to operate the locking function I wouldn't want to give it a test run. The average car I think brakes at about .6 g and a F1 car at 5 g, so I think the face modification story may have some merit.