hay simon,
might be an idea to talk to some instructors/coaches who teach .... injured/disabled people you know like paralympians and **** like that
i get a pretty fierce tennis elbow and that limits me somewhat but i have alot of excercises and about 30 less years then you i reckon and i can manage to keep doing what i do. i do alot of concreteing and jackhammering so i get some strange finger issues to and awake every morning to what i can only assume is arthritis or some ****.
rather then chaseing a light bar pressure kite tho maybe think outside the box a bit and look at how someone would kitesurf if they had NO fingers.
the finger pain im guessing is largely to do with gripping the bar and forces being excerted whilst your hands are in that position. (probably not just the kiteing that is an issue ay ay ay ay) so lets theoreticly cut off your hands and maybe have a little mini sling on each bar end....
bit like one of these ****ers

i know after a few hours of flying my fingers get sore if im gripping so i can just let my hands loosely hang in the loops.
but someone who coaches/teaches physically disabled sportsmen/ladies is going to have a far better solution. but i know this would kind of work maybe its worth tieing on a couple loops to your bar (larks head not at the 1/4 in each bar end would do it) make them reasonably loose and easy to get your hands out if under tension practice on land first with no kite attached yada yada safety safety
and yeah as long as your fingers are straight and arent under any tension or pressure i cant see why the pain wouldnt subside for a bit of relief during a session.
one question thats rather important but i wasnt aware of untill i went ****ing about in the snow and **** is IF YOU ARE WEARING GLOVES!! because if your wearing neoprene gloves and theyre DIVING GLOVES as in when the gloves are laid down and the fingers are flat, **** them off. you need special neoprene gloves that are allready curled so when laid down on the table with no hands inside the fingers of the gloves curl back to the wrist. kiteing in diving gloves will destroy your hands pretty damn fast because of the constant tension in the neoprene.