I use the pilot's seatbelt, too. It seems to work fine. Runups have nothing to do with the flight controls. Regardless of doing a run up, I always make it a practice to "wipe" the controls and call controls free and correct prior to positioning on the runway, especially if a passenger is aboard. You never know when they might have inadvertently hooked clothing, camera straps, etc., on the controls, and it is a nice gentle way of insuring that "larger" passengers know to allow some room for the flight control movement.
Vic