The 2003 Discovery has two round lights in the bumper that are "fog lights" these underpowered lights sorta throw light out down and low, but do little for seeing beyond 30 feet in front of the truck.
Granted, you do not want to be going fast when using them, but they seem rather more decorative than useful.
I have a set of 90mm Hella Projector Lowbeams laying around from a failed project. I have considered replacing the "fog lights" on the rover with these. I seems to me that they would do the same thing, throw light down low on the road with little upwards to cause glare.
What does the hive think?
Depends on how much fog you actually get. I find the times when there is fog, the low power/distance is appropriate. Which reminds me, the Miata needs a fog light...
However, putting in what you have laying around would be Hella cool! 
I run 55 watt his in BMW spec Hella projectors. If adjusted right, they have a fantastic cutoff. I'm not sure how they would work as fog lights though. They are about mid thigh as far as distance from the ground on my truck, and just create wall of white in dense fog/snow.
For fog conditions you they aren't going to be the right kind of light, they'll just scatter in the fog and make it even harder to see. If you're talking about using them for general driving in non-fog conditions, don't do it - they'll just annoy oncoming drivers.
Fog lights are a pet peeve of mine I have gone into far too many times on this forum. I will simplify my rant into this
Read this http://www.danielsternlighting.com/tech/lights/fog_lamps/fog_lamps.html
Aim them low, don't use them in the rain and treat them like high beams (IE turn em off when there is oncoming traffic) and you should be fine.
I have only ever used Fog Lamps under two circumstances.. I use them on rainy days when headlights are necessary.. and only under very extreme weather.. pouring rain and heavy snow.. and under those conditions I slow down. I actually use the rear fog more than the fronts.. But in reading the article you provided Jumper.. I think the Hella 90mms would work well. they have a sharp cut off that reflects little to no light upwards, and are a little over knee high off of the ground, so below most people's line of sight (unlike my disco's headlights)
The problem with using them in the rain is that the ground is wet providing a nice reflective surface to bounce that light up into peoples windshields that are also covered with light scattering droplets resulting in a thousand points of glare.
See if you can get Amber colored bulbs for them. Light in the yellow/amber part of the spectrum is easier on the eyes.
I keep a pair of yellow tint glasses in my car and truck for driving in poor conditions, and they help a lot. In fog, rain, snow and heavy traffic at night, I can see depth and contrast better, and they save my eyes from the shiny happy people with ultra bright blue tint headlights, and I don't get head aches, which is a problem that I've developed recently
I have a set of amber fogs for my c900 that I will try out when I get it back on the road.
the other night driving home in light rain and fog. I found the low beam on my Fiesta to work quite well. the cut off negated glare and they lit the road quite well. The high beams reached out farther but the glare was awful.
my other thoughts on these.. and this is what I may do.. use them as auxiliary headlights. They are already DOT compliant as headlights.. and with the height of my rover, only a little lower than most car's headlights. Rather than using them in bad weather, I could set them to use as actual headlights and even adjust them downwards to counteract the weight of towing.
