As far as I can recall only one aircraft of the Harrier family was fitted with an SPD - G-VTOL.
It was useful in the early ski-jump trials days as a tool to obtain a genuinely stick fixed launch. It was not enough to let go of the stick as inertia effects caused it to move both ways (aft during acceleration, forward when nozzles were selected). Asking the pilot to try and and hold the thing still had a tolerance band which muddied the traces.
When using the SPD we could break out by pulling back with a modest force (a pound or three) as the clip on the front face of the stick that held the ring on the end of the wire was suitably weak. We could adjust the length of the wire to set whatever particular tailplane angle we wanted for that launch.