Or, I could have added a single node(Z to scalar) to the setup I already had and not bothered posting the question at all! I'm bloody dim sometimes!
That makes the Perlin-warper uniform along the length of the road so, no need for two different warp shaders, since the Z to scalar is infinite from East to West it'll always cross the road at the same points on each side when the road runs North to South. Still, this means it can't possibly double-back on itself(which is good in a way too because it avoids the circular warping), some larger scaled warp over this one, in the other direction should overcome that, though, and the road will then be able to double-back.
So the road should stay reasonably consistent in width, as long as the warp amount doesn't exceed the warper's scale(then it'd thin out). It certainly avoids the problem of a patch of Perlin warp catching one side of the road and not the other, which was what I was after.
* Sorry, Frank. Just clearing up my mess.