Hi Erwin,
As I know your car from shows, I must say that it looks very good. Currently you still beat me in the engine compartment (I do envy that engine and compartment), but for the rest I think I'm on an equal level.... (I refuse to do anything about the appearence of the engine as it is currently messy, but totally original. When it breaks, that's my cue)
Still, I believe cars are made to be driven, so I drive my car every day trough rain, shine, snow, hail, lightning, salt, anything really. In my opinion a Mustang is built strong enough to withstand the abuse.
I agree that in a rainy country such as the Netherlands additional rust proofing is extremely important, but still.... My car has seen plenty of all of the above (it's outside 24 hrs a day anyway) and as long as it gets it's weekly washing, it doesn't show in any area of the car.
So I guess I'm saying....stop being so paranoid about the Dutch weather...your car will survive just fine (unless you didn't rust proof it). Instead of getting all worked up by the sudden bad weather, enjoy the car. It's still great, rain or shine!
If it's the increased chance of accidents happening that bothers you, then just keep your distance and always show what you are about to do. Personally I don't really believe it's the rain that increases your chanses on having an accident, it's the fact that modern car drivers don't pay enough attention to what is goin on and don't adjust their driving to the situation.
PS. Where was that sig pic taken?