I think if I was at the stage of peeing my pants as a last option, I'd just go right at the roadside if it came down to it. You'd get dog's abuse, but I think you'd need to be pretty far from civilisation to voluntarily cart around a bladder's worth of urine in your trousers. And how hot would it need to be to dry within an acceptable time?
I know that when Jackie Chan was in training as a boy in China, anyone who needed to go to the toilet was shouted at for being lazy; they maintained that if you needed to pee, you weren't sweating enough!
But I think most of us would collapse if we had to work that hard.