Hi Dominic. I think the earliest civil HUDs were used on an aircraft called the Dassault Mercure (Like an early baby 737) by Air Inter in France for their Nuit Postale low vis procedures in the late 1960's.
After this, they seem to fall from favour in civil aviation although very much to the forfront in the military. I believe some of the later 737 NG versions may have the facility. but don't know much else.
A few years ago I did read that a couple of the larger US carriers were running a project to develop an integrated system involving HUD. At this time there was some concern over the longevity of ILS sytems due to frequency encroachment etc, and alternatives were being explored. The idea was to use GPS to get the A/C down to around a Cat 1 DH (around the 200ft/1/2 mile point), and then use a composite HUD for final guidance. The HUD would be fed from a variety of sources, low light TV, forward looking I/R, and millimetric radar( possibly using the leading edges as an enhanced aerial), all of these combined with radar reflectors, lights and thermal markers to delineate the runway.
I believe they had some success but ran into problems due to the fact that a lot of the equipment required was developed for, and in use, by the military and was still highly classified, and also , having expended vast amounts of their budgets developing this technology, the services were loath to give it away! I've heard nothing more since, but it sounded like a good way to go.
RVR used to be measured by a fireman counting visible runway lights, whilst standing on top of his fire engine (to give an approximation of flight deck height on landing). When transmissometers came into use, their measurement was factored to try to reflect the same measurement, but sometimes problems still arose in very shallow fog, with vis at 5ft being 50M but at 15ft, 10KM, but that's just aviation!
Hope this is of some assistance