Originally Posted by mrodent

Many years later.
I did find nearly what I was looking for. These are "waterproof shorts" by tenn-outdoors. Unfortunately I find that that company went into liquidation in 2019.
I went out last night in London in an absolute downpour. These tenn-outdoors trousers are not in fact what I would call 100% waterproof: when I got home the surface was in fact not totally dry to the touch and did need to be dried out. A totally water-repellent outer fabric (some kind of plastic) would simply need to be wiped down to be totally dry.
Crucially, however, they make an enormous difference to just wearing jeans and cycling in a downpour. With just jeans my thighs and knees get horribly wet and cold in such a downpour, and of course I have to keep wearing these jeans once I get where I'm going, but with these shorts it is only the shin part of my trousers which get damp: this makes a massive difference.