The Dash 8's can actually do Cat IIIA down to 200m RVR and 50ft DH using Head-Up-Guidance which requires an extra IRS to be installed. They don't have autothrottles either and the 400s have extremely sensitive power levers which I imagine to be quite tricky to handle on an ILS.
AFAIK the HGS system establishes a path in space using the ILS and then blends out the ILS signal on the last few feet to avoid any beam distortions close to the ground. It gives guidance down to the flare. (Same for the Canadair Jets and as an alternative to Autoland on the Embraer 170/190 series).
Are autothrottles actually required to be operative for autoland on other types? It wasn't on the Fokker 70/100.