Since posting the above I have had the opportunity to look at the Byrne sketch under similar conditions, ie, high magnification with moderate de-noising.
This also shows evidence of hyper-pigmentation with some differences of pattern:
(1) the whole of the forehead is darkened in Byrne, but the dark patch extends below the right eye, lower than in JSC. On the upper eyelid, the abrupt transition from dark eyelid to white crescent extending down the cheek can be made out, but the contrast is very reduced.
(2) immediately adjacent to the left eyelid there is a network of very dark spidery filaments, similar to the marking visible in JSC. Due to the angle of the head, not much of the left side of the face can be seen.
(3) in Byrne, the black spot on the point of the chin is very large, almost as wide as the mouth, and much more prominent than in JSC. The black spot under the lower lip, however, is less prominent.
(4) there is the same patch of medium hyper-pigmentation to lower-left of the mouth as in the other three sketches. In Byrne this patch contains scattered black spots.
Differences in the degree and distribution of the hyper-pigmentation from one sketch to another might be accounted for in two ways. Jane Austen may have sometimes used makeup to disguise the worst of it; and she had occasional periods of partial remission, during which some of the discoloration may have faded somewhat.
