Entry: arachnodactylic (adj.)
In context: "...Mario's head bowed under the weight of the head-mount rig and his hideously arachnodactylic fingers whitening as the fence takes his forward weight..."
Not in the OED, but from Wikipedia: Arachnodactyly ("spider fingers") or achromachia, is a condition in which the fingers are abnormally long and slender in comparison to the palm of the hand. It can be present at birth or develop in later life.
It is also seen in some cases that all or a few fingers can be bent backwards of 180 degrees.
Other:With a few misgivings I did decide to try a Google Image search. Anyways, I kind of wonder if DFW would sit and pore through medical textbooks, just hoping to find something like Arachnodactyly.
SNOOT score: 1