What relates grotesque to disability it might not be so obvious (or it might be so stereotypically obvious that it can be rejected as abusive!). But as the origins of the grotesque lie in the aesthetic discourse, then disability aesthetics should be concerned for grotesque as a form.
That's what it lies also behind Jonathan Jones's article "Character Witness - Art & caricature", he wrote for Frieze Magazine. He contemplate on many topics, but highlights Leonardo Da Vinci's admiration to grotesque faces.
In his Notebooks in the chapter "The Practice of Painting", Leonardo is giving advices to portrait and figure painters for drawing grotesque faces.
572. OF A METHOD OF KEEPING IN MIND THE FORM OF A FACE.
"If you want to acquire facility for bearing in mind the expression
of a face, first make yourself familiar with a variety of [forms of]
several heads, eyes, noses, mouths, chins and cheeks and necks and
shoulders: And to put a case: Noses are of 10 types: straight,
bulbous, hollow, prominent above or below the middle, aquiline,
regular, flat, round or pointed. These hold good as to profile. In
full face they are of 11 types; these are equal thick in the middle,
thin in the middle, with the tip thick and the root narrow, or
narrow at the tip and wide at the root; with the nostrils wide or
narrow, high or low, and the openings wide or hidden by the point;
and you will find an equal variety in the other details; which
things you must draw from nature and fix them in your mind. Or else,
when you have to draw a face by heart, carry with you a little book
in which you have noted such features; and when you have cast a
glance at the face of the person you wish to draw, you can look, in
private, which nose or mouth is most like, or there make a little
mark to recognise it again at home. Of grotesque faces I need say
nothing, because they are kept in mind without difficulty."
Another allusion of what it might Da Vinci was thinking when he was looking at exceptional and grotesque faces, can be found in a brief record in the chapter "Miscellaneous Notes"
"Giovannina, has a fantastic face,--is at Santa Caterina, at the Hospital.""
According to Giorgio Vasari in "The Lives of the Painters, Sculptors and Architects Vol.05 (of 10)" Leonardo... "(He) was so delighted when he saw certain bizarre heads of men, with the beard or hair growing naturally, that he would follow one that pleased him a whole day, and so treasured him up in idea, that afterwards,on arriving home, he drew him as if he had had him in his presence.
Of this sort there are many heads to be seen, both of women and of men, and I have several of them, drawn by his hand with the pen, in our book of drawings, which I have mentioned so many times ; such was that of Amerigo Vespucci, which is a very beautiful head of an old man drawn with charcoal, and likewise that of Scaramuccia, Captain of the Gypsies, which afterwards came into the hands of M. Donato Valdambrini of Arezzo, Canon of S. Lorenzo, left to him by Giambullari."
This sheet might be "Scaramuccia, king of the gypsies" ; others think it was a cartoon used for the head of an onlooker in a now lost painting of the Mocking of Christ.
Although it is written in Jonathan Jones's article that "Da Vinci’s notebooks contain a large number of drawings of grotesque faces; Vasari collected them, and so did a lot of other people." and that "as Sir Kenneth Clark lamented in his 1939 biography of da Vinci, ‘for three centuries these were the most typical of his works, familiar in numerous engravings. Today we find them disgusting, or at best wearisome.’" it is also written by Jean Paul Richter, commentator of Da Vinci's "Notebooks" that "Nor should I be justified if I intended to include in the literary works the well-known caricatures of human faces attributed to Leonardo-- of which, however, it may be incidentally observed, the greater number are in my opinion undoubtedly spurious. Two only have necessarily been given owing to their presence in text, which it was desired to reproduce: Vol. I page 326, and Pl. CXXII. It can scarcely be doubted that some satirical intention is conveyed by the drawing on Pl. LXIV (text No. 688)."
Grotesque image in page 326
Pl. LXIV. Emblematic Representation; From the Royal Library, Windsor Castle-see text No. 688
On this side Adam and Eve on the other;
O misery of mankind, of how many things do
you make yourself the slave for money!
[Footnote: See PI. LXIV. The figures of Adam and Eve in the clouds
here alluded to would seem to symbolise their superiority to all
Pl. CXXII Drawing of Caricatures; From the Royal Library, Windsor Castle
And in case of dispute regarding the relation of Leonardo Da Vinci and disability aesthetics, a quote in ViaRoma100.net, on the opportunity of the exhibition "Leonardo e lo Sport" held in Athens during the Olympic Games 2004, leave no doubt regarding the relation of Leonardo Da Vinci and disability.
LEONARDO PROGETTO' PROTESI PER HANDICAP
(Google translator)"The Museo Ideale Leonardo da Vinci, who designed and curated the exhibition in Athens, on this occasion indicates that the artist/scientist is interested in the Renaissance 'also studies related to the issue of physical disability. The painter of 'Mona Lisa' 'record 'stunning drawings of mechanical limbs and a therapeutic chair, in addition to those on gymnastic exercises, the functioning of muscles and the ability' sensory. Between the sheets of anatomical studies of Leonardo, says Professor Alessandro Vezzosi, director of the Museo Ideale, are among other things: outstanding design of the structural pattern of a shoulder rebuilt with 'wires' on which the artist intends to superimpose graphically first bones and muscles. Then there is, especially, the extraordinary design of a reconstructed left leg with copper wires (''As a major round, and makes' the strings of annealed copper wire and then fold under the natural effect'': cosi' specifies in a sheet of the Royal Library of Windsor dated circa 1508)."
Symbol of this exhibition was "Head satiresca", an image never exposed to the public, dated on 1508, signed by Leonardo, and exceptionally granted for the occasion by an anonymous collector.